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

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.

gcc:
2017-06-14  Boris Kolpackov  <boris@codesynthesis.com>

	* doc/cppopts.texi: Document '-' special value to -MF.

gcc/c-family:
2017-06-14  Boris Kolpackov  <boris@codesynthesis.com>

	* c-opts.c (c_common_finish): Handle '-' special value to -MF.

From-SVN: r249201