[ASTMatchers] Add support for matching the type of a friend decl.

This allows matchers like:

  friendDecl(hasType(cxxRecordDecl(...)))
  friendDecl(hasType(asString(...)))

It seems that hasType is probably the most reasonable narrowing matcher to
overload, since it is already used to narrow to other declaration kinds.

Differential Revision: https://reviews.llvm.org/D48242

Reviewers: klimek, aaron.ballman

Subscribers: cfe-commits
llvm-svn: 334930