Add flag to enable native half type

Summary:
r235215 enables support in LLVM for legalizing f16 type in the IR.  AArch64
already had support for this.  r235215 and some backend patches brought support
for ARM, X86, X86-64, Mips and Mips64.

This change exposes the LangOption 'NativeHalfType' in the command line, so the
backend legalization can be used if desired.  NativeHalfType is enabled for
OpenCL (current behavior) or if '-fnative-half-type' is set.

Reviewers: olista01, steven_wu, ab

Subscribers: cfe-commits, srhines, aemerson

Differential Revision: http://reviews.llvm.org/D9781

llvm-svn: 237406