scan-build: explicitly say "No bugs found" if there are no reports.

Patch by Martin Storsjo!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176472 91177308-0d34-0410-b5e6-96231b3b80d8

tools/scan-build/scan-build

@@ -505,6 +505,7 @@ sub Postprocess {
       Diag("Removing directory '$Dir' because it contains no reports.\n");
       system ("rm", "-fR", $Dir);
     }
+    Diag("No bugs found.\n");
     return 0;
   }