TextDiagnosticBuffer: Fix copy-paste mistake in r197856

The TextDiagnosticBuffer is meant to scrub SourceLocations as the input/output
SourceManagers may be different.

To be safe this commit restores the original behaviour though in practice
all current users seem to share a single SM.

Would be nice to replace TextDiagnosticBuffer now that more capable interfaces
like CaptureDiagnosticConsumer / StoredDiagnosticConsumer exist.