[IR] Split the attribute pool by kind. NFC (#221620)
Change Attribute::get overloads to use fine-grained hash-consing pools
(Enum/Int/String/Type), avoiding FoldingSetNodeID serialization
overhead. Use a flat array for Enum attributes, which carry no value.
The two ConstantRange kinds keep using FoldingSet: they are not used
often, and UniquingSet cannot be used with custom UniquingSetInfo
(`APInt::operator==` asserts identical BitWidth).
Aided by Opus 5
