[JIT] TrivialMemoryManager: Fail if we can't allocate memory.

TrivialMemoryManager currently doesn't check the return type of AllocateRWX --
and returns a 'null' MemoryBlock to its caller. As pointed out by Lang,
this exposes some serious issues with the MemoryManager interface. There's,
in fact, no way to report back an error to clients rather than aborting in
case memory can't be allocated. Eventually the interface will grow to support
this, but for now, fail sooner rather than later.

Differential Revision:	http://reviews.llvm.org/D13627

llvm-svn: 250350