The symptom here was a crash in getConformanceAccessPath(), but the root
cause was that the ASTMangler used an incorrect generic signature when
looking up the conformance path for a type parameter.
The mangler requires a generic signature to be set to properly mangle types
containing retroactive conformances. However, DebugInfo just used the
IRGenModule's current signature, which wasn't always the same as the
signature describing the archetypes found inside the SILFunction.
Fixes rdar://problem/81683650 / https://bugs.swift.org/browse/SR-15046.
