When building runtimes, the compiler name (e.g. clang, clang-cl) is set based on
`CMAKE_SYSTEM_NAME` passed to `llvm_ExternalProject_Add()` through `CMAKE_ARGS` argument.
This mechanism doesn't work well if the target is Windows host.
`runtime_default_target()`/`builtin_default_target()` doesn't provide a way
to specify `CMAKE_SYSTEM_NAME` and doesn't set it either.
This patch appends variables specified in `RUNTIMES_CMAKE_ARGS`/`BUILTINS_CMAKE_ARGS`
to `CMAKE_ARGS` argument of `llvm_ExternalProject_Add()` in the case of called
from `runtime_default_target()`/`builtin_default_target()` thus in particular
it allows passing CMAKE_SYSTEM_NAME whenever it is required.
Reviewed By: phosek, compnerd, plotfi
Differential Revision: https://reviews.llvm.org/D81877
-DCMAKE_ASM_COMPILER_TARGET=${TARGET_TRIPLE}
-DCMAKE_C_COMPILER_WORKS=ON
-DCMAKE_ASM_COMPILER_WORKS=ON
+ ${BUILTINS_CMAKE_ARGS}
PASSTHROUGH_PREFIXES COMPILER_RT
USE_TOOLCHAIN
${EXTRA_ARGS})
-DCMAKE_C_COMPILER_WORKS=ON
-DCMAKE_CXX_COMPILER_WORKS=ON
-DCMAKE_ASM_COMPILER_WORKS=ON
+ ${RUNTIMES_CMAKE_ARGS}
PASSTHROUGH_PREFIXES LLVM_ENABLE_RUNTIMES
${ARG_PREFIXES}
EXTRA_TARGETS ${extra_targets}