[ASTMatchers] add three matchers for dependent expressions

Summary:
The new matchers can be used to check if an expression is type-, value- or instantiation-dependent
in a templated context.
These matchers are used in a clang-tidy check and generally useful as the
problem of unresolved templates occurs more often in clang-tidy and they
provide an easy way to check for this issue.

Reviewers: aaron.ballman, alexfh, klimek

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D51880

llvm-svn: 341958