Add an AST matcher for string-literal length

Summary:
This patch is adding support for a matcher to check string literal length.

This matcher is used in clang-tidy checkers and is part of this refactoring:
  see: http://reviews.llvm.org/D19841

Reviewers: sbenza, klimek, aaron.ballman

Subscribers: alexfh, klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D19876

llvm-svn: 269274