[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151663 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed