RenderScript support in the Frontend

Summary:

Create a new Frontend LangOpt to specify the renderscript language. It
is enabled by the "-x renderscript" option from the driver.

Add a "kernel" function attribute only for RenderScript (an "ignored
attribute" warning is generated otherwise).

Make the NativeHalfType and NativeHalfArgsAndReturns LangOpts be implied
by the RenderScript LangOpt.

Reviewers: rsmith

Subscribers: cfe-commits, srhines

Differential Revision: http://reviews.llvm.org/D21198

llvm-svn: 272342