Add loc() to the dynamic registry.

Summary:
Add loc() to the dynamic registry.
Other fixes:
 - Fix the polymorphic variant value to accept an exact match, even if
   there are other possible conversions.
 - Fix specifiesTypeLoc() to not crash on an empty
   NestedNameSpecifierLoc.

Reviewers: klimek

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D2928