Use FPT::getArgTypes() instead of manually building ArrayRefs

Made significantly easier with git-clang-format.

Differential Revision: http://llvm-reviews.chandlerc.com/D947

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183694 91177308-0d34-0410-b5e6-96231b3b80d8