Ensure we are adding T : Differentiable conformance from protocol conditional conformance

[asl]

Fixes #75711

[asl]

@swift-ci please test

[asl]

Tagging @rxwei @JaapWijnen

[slavapestov]

Should the correct witnessSubs be computed in Sema? See lib/AST/RequirementEnvironment.cpp

[asl]

It seems to be calculated correctly there:
<τ_0_0 where τ_0_0 : Differentiable> is a witness generic thunk signature and the corresponding requirement-to-witness substitution map is:

(substitution_map generic_signature=<Self where Self : P>
  (substitution Self ->
    (bound_generic_struct_type decl="main.(file)[email protected]:3:8"
      (generic_type_param_type depth=0 index=0 param_kind=type)))
  (conformance type="Self"
    (normal_conformance type="Wrapper<T>" protocol="P"
      (assoc_type req="T" type="T")
      (assoc_conformance type="Self" proto="Copyable"
        (builtin_conformance type="Wrapper<T>" protocol="Copyable"))
      (assoc_conformance type="Self" proto="Escapable"
        (builtin_conformance type="Wrapper<T>" protocol="Escapable"))
      (assoc_conformance type="Self.T" proto="Differentiable"
        (abstract_conformance protocol="Differentiable"))
      (requirement "T" conforms_to "Differentiable"))))

However, witnessSubs here is:

(substitution_map generic_signature=<T where T : Copyable, T : Escapable>
  (substitution T ->
    (primary_archetype_type address=0x15c9d8c18 conforms_to="_Differentiation.(file).Differentiable" name="\xCF\x84_0_0"
      (interface_type=generic_type_param_type depth=0 index=0 param_kind=type)))
  (conformance type="T"
    (abstract_conformance protocol="Copyable"))
  (conformance type="T"
    (abstract_conformance protocol="Escapable")))

So T : Differentiable comes from conditional conformance via substitution and is not part of generic signature.