accept 'clang++ -c a.pch -o a.o' to create PCH's object file

This way should be the same like with a.pcm for modules.
An alternative way is 'clang++ -c empty.cpp -include-pch a.pch -o a.o
-Xclang -building-pch-with-obj', which is what clang-cl's /Yc does
internally.

Differential Revision: https://reviews.llvm.org/D83716