[analyzer diagnostics] start prototyping stripping PathDiagnostics of...

[analyzer diagnostics] start prototyping stripping PathDiagnostics of unnecessary cruft caused by path inlining.

This introduces a concept of a "prunable" PathDiagnosticEvent.  Currently this is a flag, but
we may evolve the concept to make this more dynamically inferred.

llvm-svn: 151663