ASTImporter: Migrate to the FileEntryRef overload of SourceManager::createFileID, NFC

[dexonsmith]

Summary

Migrate ASTImporter::Import over to using the FileEntryRef overload
of SourceManager::createFileID. No functionality change here.

[shafik]

LGTM

clang/lib/AST/ASTImporter.cpp

if (!IsBuiltin && !Cache->BufferOverridden) {
         ToIncludeLocOrFakeLoc = ToSM.getLocForStartOfFile(ToSM.getMainFileID());
 
       if (Cache->OrigEntry && Cache->OrigEntry->getDir()) {
         // FIXME: We probably want to use getVirtualFile(), so we don't hit the
         // disk again
         // FIXME: We definitely want to re-use the existing MemoryBuffer, rather
         // than mmap the files several times.
         auto Entry =
-            ToFileManager.getFile(Cache->OrigEntry->getName());
+            ToFileManager.getOptionalFileRef(Cache->OrigEntry->getName());
         // FIXME: The filename may be a virtual name that does probably not
         // point to a valid file and we get no Entry here. In this case try with
         // the memory buffer below.
         if (Entry)
           ToID = ToSM.createFileID(*Entry, ToIncludeLocOrFakeLoc,
                                    FromSLoc.getFile().getFileCharacteristic());
       }
     }