Introduce lazy computation of a BoundGenericType's substitutions.

Teach a BoundGenericType to compute its own substitutions, which
allows AST clients to create new bound generic types without the aid
of the type checker. 

This eliminates the TypeChecker::validateTypeSimple() abomination as
well as the need for the BoundGenericType AST validation step. There
is still more cleanup to do in this area.

Note that BoundGenericType::getSubstitutions() now accepts a module
parameter, which is the place from which we will look for
conformances. This is a baby step toward properly modeling the
conformances as part of the bound generic type, and is nowhere near
complete.


Swift SVN r8193

lib/AST/Module.cpp

@@ -366,6 +366,14 @@ void Module::getTopLevelDecls(SmallVectorImpl<Decl*> &Results) {
 ArrayRef<Substitution> BoundGenericType::getSubstitutions(
                                            Module *module,
                                            LazyResolver *resolver) {
+  // FIXME: If there is no module, infer one. This is a hack for callers that
+  // don't have access to the module. It will have to go away once we're
+  // properly differentiating bound generic types based on the protocol
+  // conformances visible from a given module.
+  if (!module) {
+    module = getDecl()->getParentModule();
+  }
+
   // If we already have a cached copy of the substitutions, return them.
   auto *canon = getCanonicalType()->castTo<BoundGenericType>();
   const ASTContext &ctx = canon->getASTContext();