TableGen: Fix ASAN error

TableGen: Fix ASAN error

Summary:
As a bonus, this arguably improves the code by making it simpler.

gcc 8 on Ubuntu 18.10 reports the following:

==39667==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7fffffff8ae0 at pc 0x555555dbfc68 bp 0x7fffffff8760 sp 0x7fffffff8750
WRITE of size 8 at 0x7fffffff8ae0 thread T0
    #0 0x555555dbfc67 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_Alloc_hider::_Alloc_hider(char*, std::allocator<char>&&) /usr/include/c++/8/bits/basic_string.h:149
    #1 0x555555dbfc67 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&) /usr/include/c++/8/bits/basic_string.h:542
    #2 0x555555dbfc67 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&) /usr/include/c++/8/bits/basic_string.h:6009
    #3 0x555555dbfc67 in searchableFieldType /home/nha/amd/build/san/llvm-src/utils/TableGen/SearchableTableEmitter.cpp:168
    (...)

Address 0x7fffffff8ae0 is located in stack of thread T0 at offset 864 in frame
    #0 0x555555dbef3f in searchableFieldType /home/nha/amd/build/san/llvm-src/utils/TableGen/SearchableTableEmitter.cpp:148

Reviewers: fhahn, simon_tatham, kparzysz

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53931

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345749 91177308-0d34-0410-b5e6-96231b3b80d8