Merge Swift & Clang VFS in the ClangImporter

The clang importer has to deal with two virtual file systems, one coming
from clang, and one coming from swift. Currently, if both are set, we
emit a diagnostic that we'll pick the swift one.

This commit changes that, by merging the two virtual file systems. The
motivation for this change is the reproducer infrastructure in LLDB,
which adds a third virtual file system to the mix.

tools/SourceKit/lib/SwiftLang/SwiftASTManager.cpp

@@ -950,7 +950,6 @@ ASTUnitRef ASTProducer::createASTUnit(
 
   if (fileSystem != llvm::vfs::getRealFileSystem()) {
     CompIns.getSourceMgr().setFileSystem(fileSystem);
-    Invocation.getClangImporterOptions().ForceUseSwiftVirtualFileSystem = true;
   }
 
   if (CompIns.setup(Invocation)) {