[clang][modules] Handle explicit modules when checking for .Private -> _Private

While we eventually want to remove the mapping from .Private to _Private
modules, until we do, ensure that it behaves the same for explicit
modules.

rdar://107449872

Differential Revision: https://reviews.llvm.org/D147477

(cherry picked from commit 8ec36e6)

Author: benlangmuir

clang/lib/Frontend/CompilerInstance.cpp

@@ -2081,8 +2081,12 @@ CompilerInstance::loadModule(SourceLocation ImportLoc,
           PrivateModule, PP->getIdentifierInfo(Module->Name)->getTokenID());
       PrivPath.push_back(std::make_pair(&II, Path[0].second));
 
+      std::string FileName;
+      // If there is a modulemap module or prebuilt module, load it.
       if (PP->getHeaderSearchInfo().lookupModule(PrivateModule, ImportLoc, true,
-                                                 !IsInclusionDirective))
+                                                 !IsInclusionDirective) ||
+          selectModuleSource(nullptr, PrivateModule, FileName, BuiltModules,
+                             PP->getHeaderSearchInfo()) != MS_ModuleNotFound)
         Sub = loadModule(ImportLoc, PrivPath, Visibility, IsInclusionDirective);
       if (Sub) {
         MapPrivateSubModToTopLevel = true;

clang/test/Modules/implicit-private-with-submodule-explicit.m

@@ -0,0 +1,31 @@
+// Checks that the use of .Private to refer to _Private modules works with an
+// explicit module.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -x objective-c -fmodules -fno-implicit-modules -emit-module -fmodule-name=A %t/module.modulemap -o %t/A.pcm
+// RUN: %clang_cc1 -x objective-c -fmodules -fno-implicit-modules -emit-module -fmodule-name=A_Private %t/module.modulemap -o %t/A_Private.pcm
+
+// Check lazily-loaded module
+// RUN: %clang_cc1 -x objective-c -verify -fmodules -fno-implicit-modules -fmodule-file=A=%t/A.pcm -fmodule-file=A_Private=%t/A_Private.pcm -fsyntax-only %t/tu.m
+
+// Check eagerly-loaded module
+// RUN: %clang_cc1 -x objective-c -verify -fmodules -fno-implicit-modules -fmodule-file=%t/A.pcm -fmodule-file=%t/A_Private.pcm -fsyntax-only %t/tu.m
+
+//--- module.modulemap
+module A { header "a.h" }
+module A_Private { header "priv.h" }
+
+//--- a.h
+
+//--- priv.h
+void priv(void);
+
+//--- tu.m
+@import A.Private; // expected-warning{{no submodule named 'Private' in module 'A'; using top level 'A_Private'}}
+// expected-note@*:* {{defined here}}
+
+void tu(void) {
+  priv();
+}