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
crashes.

The comment is from f545f67d, and
was associated with manually deleting a generated module map. Then
13afbf42 added this `clearOutputFiles`
call between the comment and the code it referenced. Finally,
1f76c4e8 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.