[analyzer] Ensure that PathDiagnostics profile the same regardless of path.

PathDiagnostics are actually profiled and uniqued independently of the
path on which the bug occurred. This is used to merge diagnostics that
refer to the same issue along different paths, as well as by the plist
diagnostics to reference files created by the HTML diagnostics.

However, there are two problems with the current implementation:

1) The bug description is included in the profile, but some
   PathDiagnosticConsumers prefer abbreviated descriptions and some
   prefer verbose descriptions. Fixed by including both descriptions in
   the PathDiagnostic objects and always using the verbose one in the profile.

2) The "minimal" path generation scheme provides extra information about
   which events came from macros that the "extensive" scheme does not.
   This resulted not only in different locations for the plist and HTML
   diagnostics, but also in diagnostics being uniqued in the plist output
   but not in the HTML output. Fixed by storing the "end path" location
   explicitly in the PathDiagnostic object, rather than trying to find the
   last piece of the path when the diagnostic is requested.

This should hopefully finish unsticking our internal buildbot.

llvm-svn: 162965