Allow non-constant calls to constexpr functions. (#7067)

These turn up frequently in real-world code, for example when converting
a mutable global `Cpp.std.string_view` to a `Cpp.std.string`. Only
reject a non-constant call if the callee is `consteval`, not if it's
`constexpr`.

toolchain/check/cpp/constant.cpp

@@ -267,9 +267,12 @@ auto EvalCppCall(Context& context, SemIR::LocId loc_id,
   clang::Expr::EvalResult eval_result;
   eval_result.Diag = &notes;
   if (!call_expr->EvaluateAsConstantExpr(eval_result, context.ast_context())) {
+    if (!function_decl->isConsteval()) {
+      return SemIR::ConstantId::NotConstant;
+    }
     context.clang_sema().Diag(call_expr->getBeginLoc(),
                               clang::diag::err_invalid_consteval_call)
-        << function_decl << function_decl->isConsteval();
+        << function_decl << /*is consteval*/ true;
     for (const auto& note : notes) {
       context.clang_sema().Diag(note.first, note.second);
     }