Change the prototype for MCContext::FatalError() so it can be called
from places like MCCodeEmitter() in the MC backend when the
MCContext is const.

I was going to use this in my change for r206669 but Jim convinced
me to use an assert there. But this still is a good tweak.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206923 91177308-0d34-0410-b5e6-96231b3b80d8