Build process for variants of LLVM dependency

Brad is correct about CHPL_LLVM=bundled. In that case we compile the runtime with both the clang-included that was built in third-party and the CHPL_TARGET_COMPILER. If using --llvm we link with the clang-included runtime using clang-included. With --no-llvm we use the CHPL_TARGET_COMPILER.

For CHPL_LLVM=system we again build two versions of the runtime - one with CHPL_TARGET_COMPILER and one with the clang that is associated with the system install of llvm. If --llvm is used we link with the clang version of the runtime using the system clang. For --no-llvm we link with the CHPL_TARGET_COMPILER version of the runtime using the CHPL_TARGET_COMPILER.

David