[PR48898][CMake] Support MinGW Toolchain tool sin llvm_ExternalProject_Add

Windows is in the unique position of having two drivers, clang-cl and normal GNU clang, depending on whether a GNU or MSVC target is used. The current implementation with the USE_TOOLCHAIN argument assumes that when CMAKE_SYSTEM_NAME is set to Windows that clang-cl should be used, which is the incorrect choice when targeting a GNU environment.

This patch solves this problem by adding an optional TARGET_TRIPLE argument to llvm_ExternalProject_Add, which sets the various CMAKE_<LANG>_COMPILER_TARGET variables. Additionally, if the triple is detected as an MSVC environment, clang-cl and similar MSVC specific tools will be used instead of the GNU tools.