Tracking exception specification source locations

Summary:
We do not currently track the source locations for exception specifications such
that their source range can be queried through the AST. This leads to trying to
write more complex code to determine the source range for uses like FixItHints
(see D18575 for an example). In addition to use within tools like clang-tidy, I
think this information may become more important to track as exception
specifications become more integrated into the type system.

Patch by Don Hinton.

Reviewers: rsmith

Subscribers: malcolm.parsons, sbarzowski, alexfh, hintonda, cfe-commits

Differential Revision: https://reviews.llvm.org/D20428

llvm-svn: 291771