Compiler.h: Leave LLVM_BUILTIN_UNREACHABLE undefined if it is unavailable in host compiler.

Users of LLVM_BUILTIN_UNREACHABLE should be responsible in the case when LLVM_BUILTIN_UNREACHABLE is undefined.

Actually, (0, (p)) in LLVM_ASSUME_ALIGNED(p, a) caused thousands of warnings on g++-4.4. It was a motivation in this commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171455 91177308-0d34-0410-b5e6-96231b3b80d8