[clang-tidy] Make the named parameter check only warn on declarations if a definition is visible.

Summary:
This allows us to copy the parameter name from the definition (as a comment)
or insert /*unused*/ in both places.

Reviewers: alexfh, klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4772

llvm-svn: 214701