Frontend: Fix confusing comment at call to clearOutputFiles, NFC
Fix the comment in front of compileModuleImpl's call to
CompilerInstance::clearOutputFiles. The purpose of this call is to
delete any stray temporary files after the module generation thread
The comment is from f545f67de3a1bfdbbfad88acde5b540ce3b82f4f, and
was associated with manually deleting a generated module map. Then
13afbf42d830dd43febbeb0855aa359ca9dbfbf9 added this clearOutputFiles
call between the comment and the code it referenced. Finally,
1f76c4e8101b9beaf8f1b10a57faa80256ab2b05 started sending the generated
module map directly to the SourceManager instead of putting it on disk,
deleting the call that the comment referenced.
No functionality change.