[lld/COFF] Ignore /LTCG, /LTCG:, /LTCGOUT:, /ILK: flags

We currently complain "could not open /LTCG: no such file or directory",
which isn't very useful.  We could emit a warning when we see this flag, but
just ignoring it seems fine.

Final missing part of PR38799.

Differential Revision: https://reviews.llvm.org/D108799