swift234

AST: Fix some nested generics issues exposed by generic initializer inheritance

BoundGenericType::getSubsitutions() would only look at the bound
generic arguments of the innermost type, ignoring parent types.
However, it would then proceed to walk the AllArchetypes list
of all outer generic parameter lists when forming the final
result.

The gatherAllSubstitutions() would also walk through parent types.
As a result, outer generic parameters would appear multiple
times.

Simplify gatherAllSubstitutions() to just skip to the innermost
BoundGenericType, and delegate to getSubsitutions() for the rest.

Most calls to gatherAllSubstitutions() are from SILGen it seems,
and fix only fixes one compiler_crasher.

However an upcoming patch adds a new call to gatherAllSubstitutions()
which caused some crashers to regress, so I'm going to fix it
properly here.