Changing some strange code into an assert; NFC

This code was added in b65b1f322b, but it
was not noticed that the [[fallthrough]] behavior was very wrong. In C
mode, we would set the ParenExprType to CompoundLiteral and then
promptly overwrite that information by falling through.

After some investigation, I convinced myself that it is not possible to
hit this code path in C, only in C++. I've switched it to be an
assertion; I don't expect to hit it, but if we do hit it, that will at
least give us a code example we can use to reason about the intent of
the original code.

clang/lib/Parse/ParseExpr.cpp

@@ -943,8 +943,7 @@ ExprResult Parser::ParseCastExpression(CastParseKind ParseKind,
     ParenParseOption ParenExprType;
     switch (ParseKind) {
       case CastParseKind::UnaryExprOnly:
-        if (!getLangOpts().CPlusPlus)
-          ParenExprType = CompoundLiteral;
+        assert(getLangOpts().CPlusPlus && "not possible to get here in C");
         [[fallthrough]];
       case CastParseKind::AnyCastExpr:
         ParenExprType = ParenParseOption::CastExpr;