[analyzer] MIGChecker: Improve intermediate diagnostic notes.

Add a BugReporterVisitor for highlighting the events of deallocating a
parameter. All such events are relevant to the emitted report (as long as the
report is indeed emitted), so all of them will get highlighted.

Add a trackExpressionValue visitor for highlighting where does the error return
code come from.

Do not add a trackExpressionValue visitor for highlighting how the deallocated
argument(s) was(were) copied around. This still remains to be implemented.

rdar://problem/35380337

Differential Revision: https://reviews.llvm.org/D58368

llvm-svn: 354641