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.

llvm-svn: 206923
diff --git a/llvm/lib/MC/MCContext.cpp b/llvm/lib/MC/MCContext.cpp
index 6f0c391..601903a 100644
--- a/llvm/lib/MC/MCContext.cpp
+++ b/llvm/lib/MC/MCContext.cpp
@@ -339,7 +339,7 @@
   return !MCDwarfFiles[FileNumber].Name.empty();
 }
 
-void MCContext::FatalError(SMLoc Loc, const Twine &Msg) {
+void MCContext::FatalError(SMLoc Loc, const Twine &Msg) const {
   // If we have a source manager and a location, use it. Otherwise just
   // use the generic report_fatal_error().
   if (!SrcMgr || Loc == SMLoc())