[Driver] Add --vfsoverlay flag

This flag implies `-ivfsoverlay`, and additionally passes the same
argument to the linker if it supports it. At present the only linker
which does is lld-link, so this functionality has only been added to
the MSVC toolchain. Additionally this option has been made a
CoreOption so that clang-cl can use it without `-Xclang`

Differential Revision: https://reviews.llvm.org/D141808