[AST] Treat platform modules as non-user

Modules defined within the SDK are considered
non-user modules, extend this to any module found
within the parent platform directory if there is
one. This ensures we include modules such as
XCTest and Testing.

rdar://131854240

include/swift/AST/SearchPathOptions.h

@@ -380,6 +380,8 @@ private:
 
   std::optional<StringRef> SysRoot = std::nullopt;
 
+  mutable std::optional<std::string> SDKPlatformPath = std::nullopt;
+
 public:
   StringRef getSDKPath() const { return SDKPath; }
 
@@ -398,6 +400,12 @@ public:
     Lookup.searchPathsDidChange();
   }
 
+  /// Retrieves the corresponding parent platform path for the SDK, or
+  /// \c nullopt if there isn't one.
+  /// NOTE: This computes and caches the result, and as such will not respect
+  /// a different FileSystem being passed later.
+  std::optional<StringRef> getSDKPlatformPath(llvm::vfs::FileSystem *FS) const;
+
   std::optional<StringRef> getWinSDKRoot() const { return WinSDKRoot; }
   void setWinSDKRoot(StringRef root) {
     WinSDKRoot = root;