Add missing vendor table entries for Clang and AppleClang to identify
them.  Previously this worked only if Clang was also enabled as a C or
CXX compiler first because we used to copy the C compiler id.  However,
that was removed by commit v3.13.0-rc1~44^2 (ASM: Search for full path
even when using C or C++ compiler, 2018-09-26).

Fixes: #18575