When calculating the name to display for inline namespaces, we have
custom logic to try to hide redundant inline namespaces from the
diagnostic. Calculating these redundancies requires performing a lookup
in the parent declaration context, but that lookup should not try to
look through transparent declaration contexts, like linkage
specifications. Instead, loop up the declaration context chain until we
find a non-transparent context and use that instead.
This fixes PR49954.
except where they are necessary to disambiguate the target.
This substantially improves diagnostics from the standard library,
which are otherwise full of `::__1::` noise.
Aaron Ballman