[analyzer diagnostics] Refactor filtration for PathDiagnosticConsumers that don't support cross-file diagnostics
into a common place.  Currently enable this filtration for Plist diagnostics as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151664 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed