[FileCheck] Implement -v and -vv for tracing matches

-v prints all directive pattern matches.

-vv additionally prints info that might be noise to users but that can
be helpful to FileCheck developers.

To maximize code reuse and to make diagnostics more consistent, this
patch also adjusts and extends some of the existing diagnostics.
CHECK-NOT failures now report variables uses.  Many more diagnostics
now report the check prefix and kind of directive.

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D47114

llvm-svn: 336967