Improve recursion-breaking when resolving type witnesses.

Specifically track when we're resolving type witnesses for a given
conformance, and refuse to recursively attempt to resolve those type
witnesses. The standard library patch in rdar://problem/21789238
triggers this, and isolating it in a small test case has proven
illusive.

Swift SVN r30160

lib/AST/Module.cpp

@@ -731,6 +731,11 @@ ArrayRef<Substitution> BoundGenericType::getSubstitutions(
     ++index;
   }
 
+  // Before recording substitutions, make sure we didn't end up doing it
+  // recursively.
+  if (auto known = ctx.getSubstitutions(canon, gpContext))
+    return *known;
+
   // Copy and record the substitutions.
   auto permanentSubs = ctx.AllocateCopy(resultSubstitutions,
                                         hasTypeVariables