Don't crash if a macro expands to an erroneous expression. (#7652)

For certain kinds of error, clang's parser will succeed but produce an
expression marked as "contains error". Clang's constant evaluator
asserts if given one of those, so return early if we encounter one.

toolchain/check/cpp/macros.cpp

@@ -84,7 +84,7 @@ auto TryEvaluateMacro(Context& context, SemIR::LocId loc_id,
     parser.ConsumeAnyToken(true);
   }
 
-  if (!success) {
+  if (!success || result_expr->containsErrors()) {
     CARBON_DIAGNOSTIC(
         InCppMacroEvaluation, Error,
         "failed to parse macro Cpp.{0} to a valid constant expression",