android: build with C++14 support

android: build with C++14 support

Fixes the following building errors:

In file included from external/llvm10/lib/Support/AArch64TargetParser.cpp:14:
In file included from external/llvm10/include/llvm/Support/AArch64TargetParser.h:17:
In file included from external/llvm10/include/llvm/ADT/StringRef.h:12:
external/llvm10/include/llvm/ADT/STLExtras.h:555:49: error: no template named 'index_sequence' in namespace 'std'
  template <size_t... Ns> value_type deref(std::index_sequence<Ns...>) const {
                                           ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:560:36: error: no template named 'index_sequence' in namespace 'std'
  decltype(iterators) tup_inc(std::index_sequence<Ns...>) const {
                              ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:565:36: error: no template named 'index_sequence' in namespace 'std'
  decltype(iterators) tup_dec(std::index_sequence<Ns...>) const {
                              ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:572:46: error: no member named 'index_sequence_for' in namespace 'std'
  value_type operator*() { return deref(std::index_sequence_for<Iters...>{}); }
                                        ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:572:65: error: 'Iters' does not refer to a value
  value_type operator*() { return deref(std::index_sequence_for<Iters...>{}); }
                                                                ^
external/llvm10/include/llvm/ADT/STLExtras.h:547:41: note: declared here
template <typename ZipType, typename... Iters>
                                        ^
external/llvm10/include/llvm/ADT/STLExtras.h:575:23: error: no member named 'index_sequence_for' in namespace 'std'
    return deref(std::index_sequence_for<Iters...>{});
                 ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:575:42: error: 'Iters' does not refer to a value
    return deref(std::index_sequence_for<Iters...>{});
                                         ^
external/llvm10/include/llvm/ADT/STLExtras.h:547:41: note: declared here
template <typename ZipType, typename... Iters>
                                        ^
external/llvm10/include/llvm/ADT/STLExtras.h:579:30: error: no member named 'index_sequence_for' in namespace 'std'
    iterators = tup_inc(std::index_sequence_for<Iters...>{});
                        ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:579:49: error: 'Iters' does not refer to a value
    iterators = tup_inc(std::index_sequence_for<Iters...>{});
                                                ^
external/llvm10/include/llvm/ADT/STLExtras.h:547:41: note: declared here
template <typename ZipType, typename... Iters>
                                        ^
external/llvm10/include/llvm/ADT/STLExtras.h:586:30: error: no member named 'index_sequence_for' in namespace 'std'
    iterators = tup_dec(std::index_sequence_for<Iters...>{});
                        ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:586:49: error: 'Iters' does not refer to a value
    iterators = tup_dec(std::index_sequence_for<Iters...>{});
                                                ^
external/llvm10/include/llvm/ADT/STLExtras.h:547:41: note: declared here
template <typename ZipType, typename... Iters>
                                        ^
external/llvm10/include/llvm/ADT/STLExtras.h:606:18: error: no template named 'index_sequence' in namespace 'std'
            std::index_sequence<Ns...>) const {
            ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:618:30: error: no member named 'index_sequence_for' in namespace 'std'
    return !test(other, std::index_sequence_for<Iters...>{});
                        ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:618:49: error: 'Iters' does not refer to a value
    return !test(other, std::index_sequence_for<Iters...>{});
                                                ^
external/llvm10/include/llvm/ADT/STLExtras.h:602:23: note: declared here
template <typename... Iters>
                      ^
external/llvm10/include/llvm/ADT/STLExtras.h:635:28: error: no template named 'index_sequence' in namespace 'std'
  iterator begin_impl(std::index_sequence<Ns...>) const {
                      ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:638:50: error: no template named 'index_sequence' in namespace 'std'
  template <size_t... Ns> iterator end_impl(std::index_sequence<Ns...>) const {
                                            ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:646:28: error: no member named 'index_sequence_for' in namespace 'std'
    return begin_impl(std::index_sequence_for<Args...>{});
                      ~~~~~^
external/llvm10/include/llvm/ADT/STLExtras.h:646:47: error: 'Args' does not refer to a value
    return begin_impl(std::index_sequence_for<Args...>{});
                                              ^
external/llvm10/include/llvm/ADT/STLExtras.h:622:60: note: declared here
template <template <typename...> class ItType, typename... Args> class zippy {
                                                           ^
external/llvm10/include/llvm/ADT/STLExtras.h:648:47: error: no member named 'index_sequence_for' in namespace 'std'
  iterator end() const { return end_impl(std::index_sequence_for<Args...>{}); }
                                         ~~~~~^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.