On Apple platforms, use swiftmodule directories for the stdlib (#21797)

This changes the Swift resource directory from looking like

    lib/
      swift/
        macosx/
          libswiftCore.dylib
          libswiftDarwin.dylib
          x86_64/
            Swift.swiftmodule
            Swift.swiftdoc
            Darwin.swiftmodule
            Darwin.swiftdoc

to

    lib/
      swift/
        macosx/
          libswiftCore.dylib
          libswiftDarwin.dylib
          Swift.swiftmodule/
            x86_64.swiftmodule
            x86_64.swiftdoc
          Darwin.swiftmodule/
            x86_64.swiftmodule
            x86_64.swiftdoc

matching the layout we use for multi-architecture swiftmodules
everywhere else (particularly frameworks).

There's no change in this commit to how Linux swiftmodules are
packaged. There's been past interest in going the /opposite/ direction
for Linux, since there's not standard support for fat
(multi-architecture) .so libraries. Moving the .so search path /down/
to an architecture-specific directory on Linux would allow the same
resource directory to be used for both host-compiling and
cross-compiling.

rdar://problem/43545560

lib/Serialization/SerializedModuleLoader.cpp

@@ -301,10 +301,21 @@ SerializedModuleLoaderBase::findModule(AccessPathElem moduleID,
 
   // Search the runtime import path.
   isFramework = false;
-  return !findModuleFilesInDirectory(
-      moduleID, Ctx.SearchPathOpts.RuntimeLibraryImportPath,
-      moduleFilename.str(), moduleDocFilename.str(), moduleBuffer,
-      moduleDocBuffer);
+  currPath = Ctx.SearchPathOpts.RuntimeLibraryImportPath;
+  if (Ctx.LangOpts.Target.isOSDarwin()) {
+    // Apple platforms always use architecture-specific files within a
+    // .swiftmodule directory for the stdlib.
+    llvm::sys::path::append(currPath, moduleFilename.str());
+    return !findModuleFilesInDirectory(moduleID, currPath,
+                                       archFileNames.first,
+                                       archFileNames.second,
+                                       moduleBuffer, moduleDocBuffer);
+  }
+  // Non-Apple platforms always use single-architecture swiftmodules.
+  return !findModuleFilesInDirectory(moduleID, currPath,
+                                     moduleFilename.str(),
+                                     moduleDocFilename.str(),
+                                     moduleBuffer, moduleDocBuffer);
 }
 
 static std::pair<StringRef, clang::VersionTuple>