llvm_unreachable->llvm_unreachable(0), LLVM_UNREACHABLE->llvm_unreachable.

This adds location info for all llvm_unreachable calls (which is a macro now) in
!NDEBUG builds.
In NDEBUG builds location info and the message is off (it only prints
"UREACHABLE executed").

llvm-svn: 75640