Add PerformCallToCppFunction() which calls simplified version of PerformCppOverloadResolution() before calling PerformCallToFunction() (#6122)

Instead of calling `PerformCppOverloadResolution()` and use the complex
return value to call `PerformCallToFunction()`, we call
`PerformCallToCppFunction()` which will call both
`PerformCppOverloadResolution()` and `PerformCallToFunction()`.

Followup of #6112.
Part of #5995.

toolchain/check/cpp/operators.cpp

@@ -210,4 +210,9 @@ auto LookupCppOperator(Context& context, SemIR::LocId loc_id, Operator op,
                               /*naming_class=*/nullptr, std::move(functions));
 }
 
+auto IsCppOperatorMethodDecl(clang::Decl* decl) -> bool {
+  auto* clang_method_decl = dyn_cast<clang::CXXMethodDecl>(decl);
+  return clang_method_decl && clang_method_decl->isOverloadedOperator();
+}
+
 }  // namespace Carbon::Check