clang: Work around toolchain file use of internal CMake variables
Since commit d44c0db0b2 (clang: setup correct configuration in gnu mode,
2019-02-20, v3.15.0-rc1~41^2~5) our Clang compiler information modules
need the `CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT` variable that we
compute during compiler detection. However, some existing toolchain
files set our internal `CMAKE_<LANG>_COMPILER_ID_RUN` variables and
block that detection, but do not set the new frontend variant variable.
Help them out by setting `CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT` anyway.
Fixes: #19515, #19539
2 files changed
