[-Wunsafe-buffer-usage] Fix assert when constexpr size passed to snprintf() (llvm#119786) (llvm#124022)

EvaluateAsConstExpr() can return an lvalue which is not compatible
with a subsequent getInt() call. Instead, use EvaluateAsInt() which
will use all techniques availble to get an int result compatible
with the subsequent getInt() call.

Author: tsepez

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -930,7 +930,7 @@ AST_MATCHER(CallExpr, hasUnsafeSnprintfBuffer) {
       // The array element type must be compatible with `char` otherwise an
       // explicit cast will be needed, which will make this check unreachable.
       // Therefore, the array extent is same as its' bytewise size.
-      if (Size->EvaluateAsConstantExpr(ER, Ctx)) {
+      if (Size->EvaluateAsInt(ER, Ctx)) {
         APSInt EVal = ER.Val.getInt(); // Size must have integer type
 
         return APSInt::compareValues(EVal, APSInt(CAT->getSize(), true)) != 0;