swift234

Verify the sanity of the nested types of archetypes.

Bogus nested types of archetypes in the AST were resulting in problems
for generic specialization in SIL, captured by
rdar://problem/19907135. Add AST-level verification to catch such
problems, which should have been fixed by the recent work to deal with
substitutions into concrete types of potential archetypes.

Swift SVN r29984