Add 'enum_select' diagnostic selection to clang. (#122505)

This causes us to generate an enum to go along with the select
diagnostic, which allows for clearer diagnostic error emit lines.

The sy

Add 'enum_select' diagnostic selection to clang. (#122505)

This causes us to generate an enum to go along with the select
diagnostic, which allows for clearer diagnostic error emit lines.

The syntax for this is:

%enum_select<EnumerationName>{%OptionalEnumeratorName{Text}|{Text2}}0

Where the curley brackets around the select-text are only required if an
Enumerator name is provided.

The TableGen here emits this as a normal 'select' to the frontend, which
permits us to reuse all of the existing 'select' infrastructure.
Documentation is the same as well.

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Co-authored-by: Aaron Ballman <aaron@aaronballman.com>

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