[clang-cl] make -f(no-)ident a CoreOption

On clang emits the compiler version string into debug information
by default for both dwarf and codeview. That makes compiler output
needlessly compiler-version-dependent which makes e.g. comparing
object file outputs during a bisect hard. So it's nice if there's
an easy way to turn this off.

(On ELF, this flag also controls the .comment section, but that
part is ELF-only. The debug-info bit isn't.)

Differential Revision: https://reviews.llvm.org/D97695