[OCaml] Use LLVMCreateMessage with constant strings when calling llvm_raise

The llvm_string_of_message function, called by llvm_raise, calls
LLVMDisposeMessage, which expects the message to be dynamically
allocated; it fails freeing the message otherwise. So always
dynamically allocate with LLVMCreateMessage.

Differential Revision: http://reviews.llvm.org/D18675

llvm-svn: 265116