RFA: libstdc++ testsuite PATCH for better error message pruning

[Jason Merrill]

Periodically when I go to poke at libstdc++ tests I am frustrated by the 
libstdc++ testsuite's lack of dg-prune-output, which I have found quite 
handy in the gcc testsuite.  So I finally ported it over, along with a 
bunch of the default pruning patterns.  As a result of the latter, a lot 
of the dg-excess-errors lines in the testsuite become unnecessary.

This patch doesn't actually introduce any uses of dg-prune-output, 
that'll come later

OK for trunk?
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