Add support for `enable_if` Attribute

[Swatinem]

This is supported in llvm, since a long time it seems:

https://github.com/llvm/llvm-project/blob/026bb84bcd42b875d77b769eb5ee4c19fc2a9719/llvm/include/llvm/Demangle/ItaniumDemangle.h#L5275-L5286

> llvm-cxxfilt "__Z6strchrUa9enable_ifIXLb1EEEPKci"
strchr(char const*, int) [enable_if:true]
> llvm-cxxfilt "__Z6memchrUa9enable_ifIXLb1EEXLb1EEEPKvU17pass_object_size1ij"
memchr(void const*, int pass_object_size1, unsigned int) [enable_if:true, true]

[khuey]

Is this documented anywhere?

[Swatinem]

I don’t really find anything for this. llvm seems to hardcode the whole Ua9enable_ifI part and match on that. No idea where that is coming from.

[khuey]

'U' is defined for vendor prefixes but the 'a' immediately after that is not compatible with the spec, as far as I can tell. It should be followed by a regular <source-name> production (i.e. the 9enable_if part).