CMakeDetermineCompilerId: Always use compiler detected from IDE

When compiler id detection also provides a CMAKE_<LANG>_COMPILER
value, use it unconditionally.  It is known to be the compiler
that is actually in use by IDE builds in VS and Xcode.  Do not
let a stray cache entry try to say otherwise.
Merge topic 'enable-language-require-compiler' into next

bdd26bf CMakeDetermineCompilerId: Always use compiler detected from IDE