Fix a think-o in the condition here. =[ I would commit the test that
caught this, but I want that in a separate commit in case there is
a need to revert the actual functional bit as part of reverting other
patches. This way, the commits relating to just getting the RTTI bits in
place are separate from the functional changes that start using them.
Switch the code added in r173885 to use the new, shiny RTTI
infrastructure on MCStreamer to test for whether there is an
MCELFStreamer object available.

This is just a cleanup on the AsmPrinter side of things, moving ad-hoc
tests of random APIs to a direct type query. But the AsmParser
completely broken. There were no tests, it just blindly cast its
streamer to an MCELFStreamer and started manipulating it.

I don't have a test case -- this actually failed on LLVM's own
regression test suite. Unfortunately the failure only appears when the
stars, compilers, and runtime align to misbehave when we read a pointer
to a formatted_raw_ostream as-if it were an MCAssembler. =/

UBSan would catch this immediately.

Many thanks to Matt for doing about 80% of the debugging work here in
GDB, Jim for helping to explain how exactly to fix this, and others for
putting up with the hair pulling that ensued during debugging it.