Use clang for llvm-related project

clang/llvm 3.5 depends on libcxx's <atomic> which uses _Atomic
not available in gcc until 4.9.  Even then, the _Atomic support
is in C11 mode, not C++11 mode.

Change-Id: Iae18d5c22fa0c640b1553640ca781de7646dfb2a
1 file changed