GSB: Diagnose redundant requirements on the rebuilt signature
We rebuild a generic signature after dropping conformance requirements
made redundant by a superclass or concrete type requirement.
When rebuilding the signature, preserve the source locations of the
original requirements, and only perform diagnostics on the rebuilt
signature.
This fixes an issue where we would emit a redundant requirement
warning even though the requirement in question was not actually
redundant.
This also avoids some unnecessary work. Most of the code in
finalize() does not need to be run twice, once before and once after
rebuilding the signature. Now we only run it after rebuilding the
signature.
Note that this regresses diagnostics in one narrow case where we
would previously diagnose a conflicting concrete type requirement.
This will be fixed once concrete type diagnostics are moved over
to use the new ExplicitRequirement infrastructure, just like all
other kinds already do.
Fixes rdar://problem/77462797.
