clang-cl: Ignore /Fd silently

clang-cl: Ignore /Fd silently

While it's true that we don't create the PDB as requested on the command
line, this is a well-documented limitation. Warning about it doesn't
help people using legacy build systems with clang-cl, and it makes the
clang-cl self-host very noisy.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@230527 91177308-0d34-0410-b5e6-96231b3b80d8