Standard Library Modules: Add workaround for <format> /utf-8

[StephanTLavavej]

Thanks to C++ MVP Marc Gregoire for reporting that import std; didn't allow <format> machinery to work with /utf-8.

First, this adds test coverage to the Standard Library Modules test. We were already exercising the affected function, we just need to add a /utf-8 configuration to light up the corresponding if constexpr paths.

Second, this adds a product code workaround. As @cdacamar noted, this is internal VSO-1538698 "Better handling for non-exported friend function declarations" again, where granting friendship to templated, non-module-exported (i.e. _Ugly) functions doesn't work.

I've audited the STL and I believe we're consistently applying the workaround now. We just need to remember that friend will trigger this compiler bug.

[MarcGregoire]

Thanks for the quick reaction!

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.