Plumbing for a Virtual File System

Adds the -vfsoverlay frontend option that enables the user to pass
VFS overlay YAML files to Swift. These files define a (potentially
many-layered) virtual mapping on which we predicate a VFS.

Switch all input-based memory buffer reads in the Frontend to the new
FileSystem-based approach.

lib/Sema/SourceLoader.cpp

@@ -42,7 +42,7 @@ static FileOrError findModule(ASTContext &ctx, StringRef moduleID,
     llvm::sys::path::append(inputFilename, moduleID);
     inputFilename.append(".swift");
     llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> FileBufOrErr =
-      llvm::MemoryBuffer::getFile(inputFilename.str());
+      ctx.SourceMgr.getFileSystem()->getBufferForFile(inputFilename.str());
 
     // Return if we loaded a file
     if (FileBufOrErr)