[PATCH] Recognize '-' as special -MF argument (write to stdout)

[Boris Kolpackov]

Hi,

Sometimes it is useful to generate pre-processed output to a file and
the dependency information to stdout for further analysis/processing.
For example:

g++ -E -MD -fdirectives-only -o test.ii test.cxx

This will generate the dependency information to test.d (as per the
documentation). While changing this behavior is probably unwise, one
traditional (e.g., supported by -o) way to handle this is to recognize
the special '-' file name as an instruction to write to stdout:

g++ -E -MD -fdirectives-only -o test.ii -MF - test.cxx

Currently this will create a file named '-'. The included patch changes
this behavior to write to stdout.

Note also that Clang has supported this from at least version 3.5.

The patch should apply cleanly to trunk. I would also like to see it
backported to previous versions, if possible. If this requires any
additional work, I am willing to do it.

Thanks,
Boris
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