Avoid including FileManager.h from SourceManager.h

Most clients of SourceManager.h need to do things like turning source
locations into file & line number pairs, but this doesn't require
bringing in FileManager.h and LLVM's FS headers.

The main code change here is to sink SM::createFileID into the cpp file.
I reason that this is not performance critical because it doesn't happen
on the diagnostic path, it happens along the paths of macro expansion
(could be hot) and new includes (less hot).

Saves some includes:
    309 -    /usr/local/google/home/rnk/llvm-project/clang/include/clang/Basic/FileManager.h
    272 -    /usr/local/google/home/rnk/llvm-project/clang/include/clang/Basic/FileSystemOptions.h
    271 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Support/VirtualFileSystem.h
    267 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Support/FileSystem.h
    266 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Support/Chrono.h

Differential Revision: https://reviews.llvm.org/D75406