[CUDA] Allow parsing of host and device code simultaneously.

 * adds -aux-triple option to specify target triple
 * propagates aux target info to AST context and Preprocessor
 * pulls in target specific preprocessor macros.
 * pulls in target-specific builtins from aux target.
 * sets appropriate host or device attribute on builtins.

Differential Revision: http://reviews.llvm.org/D12917

llvm-svn: 248299