This allows us to import the table for a given impl only once, while we
can import many ImplWitness instructions with different specifics for a
generic impl.
For example in convert_facet_value_to_narrowed_facet_type.carbon we see
that a single witness table is imported for the BitAnd interface, with
multiple witnesses (for different specifics) imported and sharing the
same table.
The ImplWitnessTable now contains a back-link to the Impl the witness is
for, allowing inst namer to name that interface in the textual semir,
and allowing the interface to be found when debugging from a witness.
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Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Use it to stringify associated constant values in diagnostics. In
passing, add missing support for stringifying bool literals. Note that
there are some cases that it doesn't stringify properly, but that's not
new here; such cases could already be observed when stringifying generic
arguments.
Dana JansensandRichard Smith