clang-cl: Enable PCH flags by default.

Now that pragma comment and pragma detect_mismatch are implemented, this might
just work.

Some pragmas aren't serialized yet (from the top of my head: code_seg, bss_seg,
data_seg, const_seg, init_seg, section, vtordisp), but these are as far as I
know usually pushed and popped within the header and usually don't leak out.
If it turns out the current PCH support isn't good enough yet, we can turn it
off again.

llvm-svn: 262749