[HIP] Support -emit-llvm for device compilation

Sometimes it is useful to compile HIP device code to LLVM BC. It is not convenient to use clang -cc1 since
there are lots of options needed.

This patch allows clang driver to compile HIP device code to LLVM BC with -emit-llvm -c.

Differential Revision: https://reviews.llvm.org/D68284

llvm-svn: 373561