LWG4515 format: a and A should insert the 0x or 0X prefix

[JeffGarland]

https://cplusplus.github.io/LWG/issue4515

LWG would like LEWG to look at this issue and decide if we should proceed with a breaking change.

@inbal2l

[jwakely]

Additional context from the LWG telecon: This would be a breaking change to a C++20 feature, but we've already made changes to floating-point formatting with to_string (P2587), and are discussing more changes to floating-point formatting with format (P3505). Another option would be a new format specifier that means "hex with prefix", e.g. %p which already means that for pointers. That could be used for integer types (where it would mean the same as %x) and for floating-point types.