Add libclang capabilities to retriete template arguments from specializations.

Includes Python bindings.

Reviewed in http://reviews.llvm.org/D5621
Patch by Rob Springer



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219529 91177308-0d34-0410-b5e6-96231b3b80d8