[Glibc] Use VFS to inject modulemap into Glibc include path
This will fix modularization issues caused by the presence of Glibc and libstdc++ in a single context.
Some Glibc headers were getting hijacked by the libstdc++ module, and the decls in them were incorrectly determined to be a part of libstdc++. This caused compiler errors when trying to use those decls.
After this change, we will be able to reference Glibc headers directly from the module map, without using an additional header (`SwiftGlibc.h`).
