Mark an impossible path as unreachable to pacify GCC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191436 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CGBuiltin.cpp

@@ -1625,6 +1625,7 @@ static llvm::VectorType *GetNeonType(CodeGenFunction *CGF,
   case NeonTypeFlags::Float64:
     return llvm::VectorType::get(CGF->DoubleTy, V1Ty ? 1 : (1 << IsQuad));
   }
+  llvm_unreachable("Unknown vector element type!");
 }
 
 Value *CodeGenFunction::EmitNeonSplat(Value *V, Constant *C) {