Add 'llvm_unreachable' to passify GCC's understanding of the constraints

of several newly un-defaulted switches. This also helps optimizers
(including LLVM's) recognize that every case is covered, and we should
assume as much.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147861 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/APFloat.cpp

@@ -1172,6 +1172,7 @@ APFloat::roundAwayFromZero(roundingMode rounding_mode,
   case rmTowardNegative:
     return sign == true;
   }
+  llvm_unreachable("Invalid rounding mode found");
 }
 
 APFloat::opStatus