Fix fragile source-col-map.c test-case.

Fix fragile source-col-map.c test-case.

The test passing was dependent upon your source tree being checked out
in a directory with a long enough path, to cause the diagnostics to
wrap at the expected locations.

Use stdin instead, so that the error messages consistently use
<stdin> as the filename, and get wrapped consistently.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239009 91177308-0d34-0410-b5e6-96231b3b80d8