[APINotes] For a re-exported module, look for APINotes in the re-exporting module's apinotes file

This upstreams swiftlang#8063.

If module FooCore is re-exported through module Foo (by using
`export_as` in the modulemap), look for attributes of FooCore symbols in
Foo.apinotes file.

Swift bundles `std.apinotes` file that adds Swift-specific attributes to
the C++ stdlib symbols. In recent versions of libc++, module std got
split into multiple top-level modules, each of them is re-exported
through std. This change allows us to keep using a single modulemap file
for all supported C++ stdlibs.

rdar://121680760

Author: egorzhdan

clang/lib/APINotes/APINotesManager.cpp

@@ -221,6 +221,7 @@ APINotesManager::getCurrentModuleAPINotes(Module *M, bool LookInModule,
                                           ArrayRef<std::string> SearchPaths) {
   FileManager &FM = SM.getFileManager();
   auto ModuleName = M->getTopLevelModuleName();
+  auto ExportedModuleName = M->getTopLevelModule()->ExportAsModule;
   llvm::SmallVector<FileEntryRef, 2> APINotes;
 
   // First, look relative to the module itself.
@@ -233,6 +234,10 @@ APINotesManager::getCurrentModuleAPINotes(Module *M, bool LookInModule,
 
         APINotes.push_back(*File);
       }
+      // If module FooCore is re-exported through module Foo, try Foo.apinotes.
+      if (!ExportedModuleName.empty())
+        if (auto File = findAPINotesFile(Dir, ExportedModuleName, WantPublic))
+          APINotes.push_back(*File);
     };
 
     if (M->IsFramework) {

clang/test/APINotes/Inputs/Headers/ExportAs.apinotes

@@ -0,0 +1,5 @@
+Name: ExportAs
+Globals:
+  - Name: globalInt
+    Availability: none
+    AvailabilityMsg: "oh no"

clang/test/APINotes/Inputs/Headers/ExportAs.h

@@ -0,0 +1 @@
+#include "ExportAsCore.h"

clang/test/APINotes/Inputs/Headers/ExportAsCore.h

@@ -0,0 +1 @@
+static int globalInt = 123;

clang/test/APINotes/Inputs/Headers/module.modulemap

@@ -2,6 +2,16 @@ module ExternCtx {
   header "ExternCtx.h"
 }
 
+module ExportAsCore {
+  header "ExportAsCore.h"
+  export_as ExportAs
+}
+
+module ExportAs {
+  header "ExportAs.h"
+  export *
+}
+
 module HeaderLib {
   header "HeaderLib.h"
 }

clang/test/APINotes/export-as.c

@@ -0,0 +1,8 @@
+// RUN: rm -rf %t && mkdir -p %t
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/ModulesCache -fdisable-module-hash -fapinotes-modules -fsyntax-only -I %S/Inputs/Headers %s -ast-dump -ast-dump-filter globalInt -x c | FileCheck %s
+
+#include "ExportAs.h"
+
+// CHECK: Dumping globalInt:
+// CHECK: VarDecl {{.+}} imported in ExportAsCore globalInt 'int'
+// CHECK: UnavailableAttr {{.+}} <<invalid sloc>> "oh no"