Fix analyzer breakage introduced by r49213: http://llvm.org/viewvc/llvm-project?rev...

Fix analyzer breakage introduced by r49213: http://llvm.org/viewvc/llvm-project?rev=49213&view=rev

The problem is that some clients of gcc lookout at the stdout output of gcc;
this requires that all the analyzer diagnostics go to stderr to avoid polluting
stdout.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@49355 91177308-0d34-0410-b5e6-96231b3b80d8