[IDE] Teach type checker about conditional conformance extensions.

Before conditional conformances, the archetypes in conformance
extensions (i.e. extension Foo: SomeProtocol) were equivalent to those
in the type decl, with the same protocol bounds and so on. The code for
printing "synthesized" members relied on this fact. This commit teaches
that code to deal with archetypes in the conditional conformance
extension when required.

Fixes rdar://problem/36553066 and SR-6930.

include/swift/AST/ASTContext.h

@@ -174,10 +174,6 @@ public:
 };
 
 class SILLayout; // From SIL
-/// \brief Describes either a nominal type declaration or an extension
-/// declaration.
-typedef llvm::PointerUnion<NominalTypeDecl *, ExtensionDecl *>
-  TypeOrExtensionDecl;
 
 /// ASTContext - This object creates and owns the AST objects.
 /// However, this class does more than just maintain context within an AST.