[COFF] Add vfsoverlay flag

This patch adds a new flag vfsoverlay similar to clang’s
ivfsoverlay flag. This is helpful when compiling on case
sensitive file systems when cross compiling to Windows.
Particularly when compiling third party code containing
\#pragma comment(“linker”, “/defaultlib:...”) which
can’t be easily changed.

Differential Revision: https://reviews.llvm.org/D125800