This commit pulls apart the inherent attribute dependence of classes
like EnumAttrInfo and EnumAttrCase, factoring them out into simpler
EnumCase and EnumInfo variants. This allows specifying the cases of an
enum without needing to make the cases, or the EnumInfo itself, a
subclass of SignlessIntegerAttrBase.
The existing classes are retained as subclasses of the new ones, both
for backwards compatibility and to allow attribute-specific information.
In addition, the new BitEnum class changes its default printer/parser
behavior: cases when multiple keywords appear, like having both nuw and
nsw in overflow flags, will no longer be quoted by the operator<<, and
the FieldParser instance will now expect multiple keywords. All
instances of BitEnumAttr retain the old behavior.
This moves the EnumAttrCase and EnumAttr classes from Attribute.h/.cpp
to a new EnumInfo.h/cpp and renames them to EnumCase and EnumInfo,
respectively.
This doesn't change any of the tablegen files or any user-facing aspects
of the enum attribute generation system, just reorganizes code in order
to make main PR (#132148) shorter.
