Add voidType() matcher.

Summary: Add voidType() matcher.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D6656

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224250 91177308-0d34-0410-b5e6-96231b3b80d8