[ASTMatchers] add matcher for decltypeType and its underlyingType

Summary:
This patch introduces a new matcher for `DecltypeType` and its underlying type
in order to fix a bug in clang-tidy, see https://reviews.llvm.org/D48717 for more.

Reviewers: aaron.ballman, alexfh, NoQ, dcoughlin

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48759

llvm-svn: 337703