[Name lookup] Add a staging flag for ASTScope-based name lookup.

Author: DougGregor

include/swift/Basic/LangOptions.h

@@ -141,6 +141,9 @@ namespace swift {
     /// new enough?
     bool EnableTargetOSChecking = true;
 
+    /// Should we use \c ASTScope-based resolution for unqualified name lookup?
+    bool EnableASTScopeLookup = false;
+
     /// Whether to use the import as member inference system
     ///
     /// When importing a global, try to infer whether we can import it as a

include/swift/Option/FrontendOptions.td

@@ -102,6 +102,9 @@ def disable_target_os_checking :
   Flag<["-"], "disable-target-os-checking">,
   HelpText<"Disable checking the target OS of serialized modules">;
 
+def enable_astscope_lookup : Flag<["-"], "enable-astscope-lookup">,
+  HelpText<"Enable ASTScope-based unqualified name lookup">;
+
 def print_clang_stats : Flag<["-"], "print-clang-stats">,
   HelpText<"Print Clang importer statistics">;
 

include/swift/Parse/Parser.h

@@ -541,6 +541,9 @@ class Parser {
   }
 
   ValueDecl *lookupInScope(DeclName Name) {
+    if (Context.LangOpts.EnableASTScopeLookup)
+      return nullptr;
+
     return getScopeInfo().lookupValueName(Name);
   }
 

lib/AST/ASTScope.cpp

@@ -1568,7 +1568,7 @@ DeclContext *ASTScope::getDeclContext() const {
 
   case ASTScopeKind::Accessors:
     // FIXME: Somewhat odd modeling because Subscripts don't have their
-    // own nodes. Maybe that should.
+    // own nodes. Maybe they should.
     if (auto subscript = dyn_cast<SubscriptDecl>(abstractStorageDecl))
       return subscript;