[ASTMatchers] New matcher hasReturnValue added

Summary: A checker (will be uploaded after this patch) needs to check implicit casts. Existing generic matcher "has" ignores implicit casts and parenthesized expressions and no specific matcher for matching return value expression preexisted. The patch adds such a matcher (hasReturnValue).

Reviewers: klimek, sbenza

Subscribers: xazax.hun, klimek, cfe-commits

Patch by Ádám Balogh!

Differential Revision: http://reviews.llvm.org/D17986

llvm-svn: 264037