[clang] Adapt ASTMatcher to explicit(bool) specifier

Summary:
Changes:
 - add an ast matcher for deductiong guide.
 - allow isExplicit matcher for deductiong guide.
 - add hasExplicitSpecifier matcher which give access to the expression of the explicit specifier if present.

Reviewers: klimek, rsmith, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: aaron.ballman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61552

llvm-svn: 363855