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`).
This logic is going to be extended to inject the modulemap and the header of Glibc in addition to libstdc++. It doesn't depend on the rest of ClangImporter, so let's extract it to a separate file to make incremental builds and IDE features faster.
