Utilize TypeReprs for type checking.

-Refactor Parser to stop creating types
-Refactor TypeChecker to create types by resolving TypeReprs.
-Remove "validation" bit from the type system.
  We don't need to "validate" every type that gets created but there's still a validation bit in TypeLoc,
  necessary because of generic substitutions.

Swift SVN r6326

lib/AST/ASTContext.cpp

@@ -851,5 +851,5 @@ ExprHandle *ExprHandle::get(ASTContext &Context, Expr *E) {
 }
 
 void TypeLoc::setInvalidType(ASTContext &C) {
-  T = ErrorType::get(C);
+  TAndValidBit.setPointerAndInt(ErrorType::get(C), true);
 }