[clang-tidy] Lift parsing of sequence of names functions to utils.

Summary:
Lift some common code used by multiple checkers.

This function is also used by checkers that are coming.
It is quite common for a checker to parse a list of names.

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19846

llvm-svn: 269065