Add -fnative-half-arguments-and-returns

Add -fnative-half-arguments-and-returns

Summary:
r246764 handled __fp16 arguments and returns for AAPCS, but skipped this
handling for OpenCL.  Simlar to OpenCL, RenderScript also handles __fp16
type natively.

This patch adds the -fnative-half-arguments-and-returns command line
flag to allow such languages to skip this coercion of __fp16.

Reviewers: srhines, olista01

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18138

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263795 91177308-0d34-0410-b5e6-96231b3b80d8