[FileCheck] Annotate input dump (final tweaks)

Apply final suggestions from probinson for this patch series plus a
few more tweaks:

* Improve various docs, for MatchType in particular.

* Rename some members of MatchType.  The main problem was that the
  term "final match" became a misnomer when CHECK-COUNT-<N> was
  created.

* Split InputStartLine, etc. declarations into multiple lines.

Differential Revision: https://reviews.llvm.org/D55738

Reviewed By: probinson

llvm-svn: 349425