Disable std::exception formatter when exceptions are disabled.

[cjacek]

Fixes building with Clang 20 and older with -fno-exceptions, which treats try/catch usage as an error.

Fixes FEX builds: FEX-Emu/FEX#5870

[vitaut]

Thanks for the PR, but I don’t think we should disable the std::exception formatter itself. Instead, please disable only the minimal part that relies on try/catch. Also, could you post the full compiler error message?

[cjacek]

Thanks for the review. The new version disables only minimal part of std::exception formatter. for exception_ptr, it seems that there would be nothing left if I did that. I also considered using FMT_TRY/FMT_CATCH, but that would just be broken with disabled exceptions.

The error message is:

fmt/include/fmt/std.h:685:9: error: cannot use 'try' with exceptions disabled
fmt/include/fmt/std.h:704:5: error: cannot use 'try' with exceptions disabled

Here is a compiler explorer repro: https://godbolt.org/z/7jKncahEK

It's specific to usage in templates that's now allowed. Outside templates it's always an error.

[vitaut]

Presence of std::exception_ptr doesn't depend on whether exceptions are enabled or disabled and in general we prefer not to make formatter availability depend on configuration. Let's move this to the format function and only guard try/catch block there giving "unknown exception" if exceptions are disabled.