(Trivial) TargetPassConfig: assert when TargetMachine has no MCAsmInfo

Summary:
This is a pretty trivial, but I thought it was worth just checking that nobody feels it's completely the wrong thing to be doing.

The motivation is that when starting a new backend, you often start with a minimal stub, pretty much just FooTargetMachine and FooTargetInfo. Once that's built, you might naturally try `llc -march=foo myinput.ll` and it seems more developer-friendly if this ends up asserting due to the lack of MCAsmInfo with an informative message rather than just segfaulting.

Reviewers: MatzeB, chandlerc

Subscribers: bogner, llvm-commits

Differential Revision: https://reviews.llvm.org/D23443

llvm-svn: 279061