testsuite: prune -freport-bug output

testsuite: prune -freport-bug output

When the compiler defaults to -freport-bug, a few dg-ice tests fail
with:

Excess errors:
Preprocessed source stored into /tmp/cc6hldZ0.out file, please attach this to your bugreport.

We could add -fno-report-bug to those tests.  But it seems to me that a
better fix would be to prune the "Preprocessed source stored..." message
in prune_gcc_output.

gcc/testsuite/ChangeLog:

	* lib/prune.exp (prune_gcc_output): Also prune -freport-bug output.

Reviewed-by: Jakub Jelinek <jakub@redhat.com>