[mlir][ods] Extend the EnumAttr tablegen class to support BitEnum attributes

This diff allows the EnumAttr class to be used for bit enum attributes (in
addition to previously supported integer enum attributes). While integer
and bit enum attributes share many common implementation aspects, parsing
bit enum values requires a separate implementation. This is accomplished
by creating empty parser and printer strings in the EnumAttrInfo record,
and having derived classes (specific to bit and integer enums) override with
an appropriate parser/printer string.

To support existing bit enums that may use a vertical bar separator, the
parser is modified to support the | token.

Tests were added for bit enums alongside integer enums.

Future diffs for fastmath attributes in the arithmetic dialect will use these
changes.

(resubmission of earlier abaondoned diff, updated to reflect subsequent changes
in the repository)

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D123880