Fix altivec regression caused by D115670 in Vec Const Eval

The Vector Constant Evaluator assumes that all the types of its
sub-expressions are going to be Vector APValues, which holds for most
situations.  However, in the 1 examples of Altivec C compilation of
operator ++ (not allowed for other vector types), the result is an
LValue.

Since the operator isn't supported for constant evaluation anyway, this
patch just fails-out of constant eval if we are in a situation where the
operand to the unary operator causes an LValue.

clang/lib/AST/ExprConstant.cpp

@@ -10434,6 +10434,15 @@ bool VectorExprEvaluator::VisitUnaryOperator(const UnaryOperator *E) {
   if (!Evaluate(SubExprValue, Info, SubExpr))
     return false;
 
+  // FIXME: This vector evaluator someday needs to be changed to be LValue
+  // aware/keep LValue information around, rather than dealing with just vector
+  // types directly. Until then, we cannot handle cases where the operand to
+  // these unary operators is an LValue. The only case I've been able to see
+  // cause this is operator++ assigning to a member expression (only valid in
+  // altivec compilations) in C mode, so this shouldn't limit us too much.
+  if (SubExprValue.isLValue())
+    return false;
+
   assert(SubExprValue.getVectorLength() == VD->getNumElements() &&
          "Vector length doesn't match type?");
 

clang/test/Sema/altivec-init.c

@@ -45,3 +45,16 @@ void test()
   int res = vGCC > vAltiVec;
   vAltiVec = 0 ? vGCC : vGCC;
 }
+
+typedef struct VecMem {
+  vector signed vec;
+} VecMem;
+
+// The following should not assert.  See qiongsiwu1's comment here: 
+// https://reviews.llvm.org/D115670
+void test2() {
+  vector signed local_vec = {1, 2, 3, 4};
+  VecMem VM;
+  VM.vec = ++local_vec;
+}
+