[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

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@214701 91177308-0d34-0410-b5e6-96231b3b80d8
[clang-tidy] Fix unnamed parameter comment insertion with multiple parameters.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@214703 91177308-0d34-0410-b5e6-96231b3b80d8