[RenderScript] Use LLVM DWARF language enum

A DWARF language vender extension for RenderScript was added to LLVM in r259348(http://reviews.llvm.org/D16409)
We should use this generated enum instead of the hardcoded value.

RenderScript is also based on C99 with some extensions, so we want to use ClangASTContext when RS is detected.

Reviewers:  clayborg
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D16766

llvm-svn: 259634