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

llvm-svn: 263795