clang-cl: print diagnostics as "error(clang): foo" in /fallback mode

This solves two problems:

1) MSBuild will not flag the build as unsuccessful just because we print
   an error in the output, since "error(clang):" doesn't seem to match
   the regex it's using.

2) It becomes more clear that the diagnostic is coming from clang as
   supposed to cl.exe.

Differential Revision: http://llvm-reviews.chandlerc.com/D1735

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191250 91177308-0d34-0410-b5e6-96231b3b80d8