Delete the output streams coming from
CompilerInstance::createOutputFile() and friends once writes are
finished. Concretely, replacing OS->flush() with OS = nullptr in:
- cc1_main()'s support for -ftime-trace`
This fixes theoretical bugs related to proxy streams, which may have
cleanups to run in their destructor. E.g., buffer_ostream supports
pwrite() by holding all content in a buffer until destruction time.
This also protects against some logic bugs, triggering a null
dereference on a latter attempt to write to the stream.
No tests, since in practice these particular code paths don't seem to
ever use buffer_ostream; you need to be writing a binary file to a
pipe (such as stdout) to hit it; but I have some other patches in the
works that add some safety, crashing if the stream hasn't been
destructed by the time the CompilerInstance is told to keep the output