[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@334930
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