[ClangImporter] Don't put forward-declared structs in the lookup table.

Otherwise, the lookup table for "CGColor" has two entries, because of
this:

    typedef struct CGColor *CGColorRef;

and that interferes with our ability to import things as members of
"CGColor" (as opposed to "CGColorRef"), which affects the fix-its we
generate when you try to use the non-member form.

This isn't necessarily the best long-term solution (as noted in the
FIXME) but it is expedient and won't break any current users.

More rdar://problem/26347297

Author: jrose-apple

lib/ClangImporter/ClangImporter.cpp

@@ -775,6 +775,24 @@ void ClangImporter::Implementation::addEntryToLookupTable(
   // Determine whether this declaration is suppressed in Swift.
   if (shouldSuppressDeclImport(named)) return;
 
+  // Leave incomplete struct/enum/union types out of the table; Swift only
+  // handles pointers to them.
+  // FIXME: At some point we probably want to be importing incomplete types,
+  // so that pointers to different incomplete types themselves have distinct
+  // types. At that time it will be necessary to make the decision of whether
+  // or not to import an incomplete type declaration based on whether it's
+  // actually the struct backing a CF type:
+  //
+  //    typedef struct CGColor *CGColorRef;
+  //
+  // The best way to do this is probably to change CFDatabase.def to include
+  // struct names when relevant, not just pointer names. That way we can check
+  // both CFDatabase.def and the objc_bridge attribute and cover all our bases.
+  if (auto *tagDecl = dyn_cast<clang::TagDecl>(named)) {
+    if (!tagDecl->getDefinition())
+      return;
+  }
+
   // If we have a name to import as, add this entry to the table.
   if (auto importedName = importFullName(named, None, &clangSema)) {
     table.addEntry(importedName.Imported, named, importedName.EffectiveContext);

test/IDE/Inputs/custom-modules/ImportAsMemberC.h

@@ -1,6 +1,6 @@
 typedef const void *CFTypeRef __attribute__((objc_bridge(id)));
 
-typedef const struct __attribute__((objc_bridge(id))) __CCPowerSupply *CCPowerSupplyRef;
+typedef const struct __attribute__((objc_bridge(id))) CCPowerSupply *CCPowerSupplyRef;
 typedef const struct __attribute__((objc_bridge(id))) __CCRefrigerator *CCRefrigeratorRef;
 typedef struct __CCRefrigerator *CCMutableRefrigeratorRef;
 

test/IDE/dump_swift_lookup_tables_objc.swift

@@ -44,8 +44,6 @@
 // CHECK-NEXT:     TU: SNSomeProtocol
 // CHECK:        UIActionSheet:
 // CHECK-NEXT:     TU: UIActionSheet
-// CHECK-NEXT:   __CCItem:
-// CHECK-NEXT:     TU: __CCItem
 // CHECK-NEXT:   __swift:
 // CHECK-NEXT:     TU: __swift
 // CHECK-NEXT:   accessibilityFloat: