[Support] Introduce formatv variant of createStringError (#80493)

Many times I have found myself wanting to create a StringError with the
ability to interpolate a StringRef into the error string. This can be
achieved with:

StringRef Foo("...");
auto Err = createStringError(..., "Something went wrong: %s",
Foo.str().c_str());

However, this requires us to construct a temporary std::string (which
may perform a memory allocation if large enough).

I propose a new variant of `createStringError` called
`createStringErrorV` which uses `formatv` under the hood. This allows
the above example to become:

StringRef Foo("...");
auto Err = createStringErrorV(..., "Something went wrong: {0}", Foo);