[asan] Unconditionally enable GC of globals on COFF.

This change restores pre-r301225 behavior, where linker GC compatible global
instrumentation was used on COFF targets disregarding -f(no-)data-sections and/or
/Gw flags.

This instrumentation puts each global in a COMDAT with an ASan descriptor for that global.
It effectively enables -fdata-sections, but limits it to ASan-instrumented globals.

llvm-svn: 301374