Triggering -save-temps from the front-end code

[Florian Weimer]

I've got some instrumentation for logging errors to a magic directory,
so that I can see if a build triggered them even when it did not
fail—and hid all compiler errors and warnings.

Unfortunately, some build systems immediately delete the input source
files.  Is there some easy way I can dump the pre-processed and
non-preprocessed sources to my log file?  I tried to understand how
-save-temps for crash recovery works, but it seems that this runs
outside of the frontend, in the driver.

Here's an example of the
challenges this would help to solve:

  Building in strict C99 mode
  <https://github.com/ksh93/ksh/issues/587>

Thanks,
Florian