swift234

Don't force implied protocol conformances into the current DeclContext.

When we check a protocol conformance, we recurse to check the implied
protocol conformances for inherited protocols first. When doing so, we
were passing down the current DeclContext, which would force the
creation of a new conformance to that protocol within that
DeclContext. This isn't what we want: we want to find or create the
conformance in whichever context it naturally belongs.

This is a partial step toward solving the problem, which eliminates
the duplicate witness tables from the example in
rdar://problem/18182969. However, we're still not using the
conformance lookup table to decide where the witness tables/protocol
conformances go, which means the actual declaration context for a
witness table is still a bit ad hoc.

Baby steps.

Swift SVN r26129