Preprocessor: support defined() with operator names for MS compatibility

Also flesh out missing tests, improve diagnostic QOI and fix a couple of corner
cases found in the process.

Fixes PR10606.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209276 91177308-0d34-0410-b5e6-96231b3b80d8