[libclang] Support for querying the exception specification type through libclang

Summary: This patch exposes the exception specification type (noexcept,
etc.) of a C++ function through libclang and Python clang.cindex.

Reviewers: rsmith, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: jbcoe, cfe-commits

Differential Revision: https://reviews.llvm.org/D34091

Patch by Andrew Bennieston

llvm-svn: 306483