Remove dead code for local declarations hash.

This is disabled for ~6 years now and never worked apparently.
If we want to speed up the DUContext, we should start by first
understanding the actual problems, then write benchmarks and
cleanup the code, instead of trying to fixup old code that never
quite worked correctly apparently.
Add QualifiedIdentifier::indexedAt.

When all we need is an IndexedIdentifier, currently we'd always had
to deserialize the Indexed one, and then serialize it again. Now,
we have direct access.
Fix compiler warning.
Move Checker struct out of function definition and cleanup code.
Further cleanup the code, esp. indentation and such.
Also add indexedFirst and indexedLast.