Frontend/VerifyDiagnosticConsumer.cpp: Fix an expression that had side-effect.

It had been causing test "Misc/diag-verify.cpp" failure on ms cl.exe. The emission was ordered unexpectedly as below;

First)  error: 'error' diagnostics seen but not expected:
Second) error: 'error' diagnostics expected but not seen:

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@146830 91177308-0d34-0410-b5e6-96231b3b80d8