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
diff --git a/test/Driver/cl-fallback.c b/test/Driver/cl-fallback.c
index ce7e245..1a0d7e0 100644
--- a/test/Driver/cl-fallback.c
+++ b/test/Driver/cl-fallback.c
@@ -6,6 +6,7 @@
 
 // RUN: %clang_cl /fallback /Dfoo=bar /Ubaz /Ifoo /O0 /Ox /GR /GR- /LD /LDd \
 // RUN:     /MD /MDd /MTd /MT -### -- %s 2>&1 | FileCheck %s
+// CHECK: "-fdiagnostics-format" "msvc-fallback"
 // CHECK: ||
 // CHECK: cl.exe
 // CHECK: "/c"