[APINotes] Support globals in extern "C++" blocks

This will be used to apply attributes to the C++ stdlib functions like `strstr`, `exit`, etc.

rdar://114382260
(cherry picked from commit 2e44311)

Author: egorzhdan

clang/lib/Sema/SemaAPINotes.cpp

@@ -812,7 +812,8 @@ void Sema::ProcessAPINotes(Decl *D) {
 
   // Globals.
   if (D->getDeclContext()->isFileContext() ||
-      D->getDeclContext()->isExternCContext()) {
+      D->getDeclContext()->isExternCContext() ||
+      D->getDeclContext()->isExternCXXContext()) {
     // Global variables.
     if (auto VD = dyn_cast<VarDecl>(D)) {
       for (auto Reader : APINotes.findAPINotes(D->getLocation())) {

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

@@ -0,0 +1,15 @@
+Name: ExternCtx
+Globals:
+  - Name: globalInExternC
+    Availability: none
+    AvailabilityMsg: "oh no"
+  - Name: globalInExternCXX
+    Availability: none
+    AvailabilityMsg: "oh no #2"
+Functions:
+  - Name: globalFuncInExternC
+    Availability: none
+    AvailabilityMsg: "oh no #3"
+  - Name: globalFuncInExternCXX
+    Availability: none
+    AvailabilityMsg: "oh no #4"

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

@@ -0,0 +1,11 @@
+extern "C" {
+  static int globalInExternC = 1;
+
+  static void globalFuncInExternC() {}
+}
+
+extern "C++" {
+  static int globalInExternCXX = 2;
+
+  static void globalFuncInExternCXX() {}
+}

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

@@ -1,3 +1,7 @@
+module ExternCtx {
+  header "ExternCtx.h"
+}
+
 module HeaderLib {
   header "HeaderLib.h"
 }

clang/test/APINotes/extern-context.cpp

@@ -0,0 +1,23 @@
+// 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 globalInExternC -x c++ | FileCheck -check-prefix=CHECK-EXTERN-C %s
+// 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 globalInExternCXX -x c++ | FileCheck -check-prefix=CHECK-EXTERN-CXX %s
+// 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 globalFuncInExternC -x c++ | FileCheck -check-prefix=CHECK-FUNC-EXTERN-C %s
+// 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 globalFuncInExternCXX -x c++ | FileCheck -check-prefix=CHECK-FUNC-EXTERN-CXX %s
+
+#include "ExternCtx.h"
+
+// CHECK-EXTERN-C: Dumping globalInExternC:
+// CHECK-EXTERN-C: VarDecl {{.+}} imported in ExternCtx globalInExternC 'int'
+// CHECK-EXTERN-C: UnavailableAttr {{.+}} <<invalid sloc>> "oh no"
+
+// CHECK-EXTERN-CXX: Dumping globalInExternCXX:
+// CHECK-EXTERN-CXX: VarDecl {{.+}} imported in ExternCtx globalInExternCXX 'int'
+// CHECK-EXTERN-CXX: UnavailableAttr {{.+}} <<invalid sloc>> "oh no #2"
+
+// CHECK-FUNC-EXTERN-C: Dumping globalFuncInExternC:
+// CHECK-FUNC-EXTERN-C: FunctionDecl {{.+}} imported in ExternCtx globalFuncInExternC 'void ()'
+// CHECK-FUNC-EXTERN-C: UnavailableAttr {{.+}} <<invalid sloc>> "oh no #3"
+
+// CHECK-FUNC-EXTERN-CXX: Dumping globalFuncInExternCXX:
+// CHECK-FUNC-EXTERN-CXX: FunctionDecl {{.+}} imported in ExternCtx globalFuncInExternCXX 'void ()'
+// CHECK-FUNC-EXTERN-CXX: UnavailableAttr {{.+}} <<invalid sloc>> "oh no #4"