clang: Work around toolchain file use of internal CMake variables
Since !2992 (merged) 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 (closed), #19539 (closed)