[AST] Do not copy SearchPathOptions

`updateNonUserModule` was accidentally copying `SearchPathOptions`. Take
a reference to it instead.

Also avoid calling `updateNonUserModule` when it isn't needed.

Resolves rdar://107155587.

Author: bnbarham

include/swift/AST/Module.h

@@ -663,7 +663,7 @@ class ModuleDecl
   /// \returns true if this module is part of the stdlib or contained within
   /// the SDK. If no SDK was specified, also falls back to whether the module
   /// was specified as a system module (ie. it's on the system search path).
-  bool isNonUserModule() const { return Bits.ModuleDecl.IsNonUserModule; }
+  bool isNonUserModule() const;
 
 private:
   /// Update whether this module is a non-user module, see \c isNonUserModule.

lib/AST/Module.cpp

@@ -609,42 +609,60 @@ ModuleDecl::ModuleDecl(Identifier name, ASTContext &ctx,
 }
 
 void ModuleDecl::setIsSystemModule(bool flag) {
+  bool previous = Bits.ModuleDecl.IsSystemModule;
+
   Bits.ModuleDecl.IsSystemModule = flag;
-  updateNonUserModule(/*newFile=*/nullptr);
+
+  if (flag && previous != flag) {
+    updateNonUserModule(/*newFile=*/nullptr);
+  }
 }
 
 static bool prefixMatches(StringRef prefix, StringRef path) {
-  auto i = llvm::sys::path::begin(prefix), e = llvm::sys::path::end(prefix);
-  for (auto pi = llvm::sys::path::begin(path), pe = llvm::sys::path::end(path);
-       i != e && pi != pe; ++i, ++pi) {
-    if (*i != *pi)
+  auto prefixIt = llvm::sys::path::begin(prefix),
+       prefixEnd = llvm::sys::path::end(prefix);
+  for (auto pathIt = llvm::sys::path::begin(path),
+            pathEnd = llvm::sys::path::end(path);
+       prefixIt != prefixEnd && pathIt != pathEnd; ++prefixIt, ++pathIt) {
+    if (*prefixIt != *pathIt)
       return false;
   }
-  return i == e;
+  return prefixIt == prefixEnd;
+}
+
+bool ModuleDecl::isNonUserModule() const {
+  return const_cast<ModuleDecl *>(this)
+      ->getTopLevelModule()
+      ->Bits.ModuleDecl.IsNonUserModule;
 }
 
 void ModuleDecl::updateNonUserModule(FileUnit *newUnit) {
   if (isNonUserModule())
     return;
 
-  SearchPathOptions searchPathOpts = getASTContext().SearchPathOpts;
+  // If there's no SDK path, fallback to checking whther the module was
+  // in the system search path or a clang system module.
+  SearchPathOptions &searchPathOpts = getASTContext().SearchPathOpts;
   StringRef sdkPath = searchPathOpts.getSDKPath();
-
-  if (isStdlibModule() || (sdkPath.empty() && isSystemModule())) {
+  if (sdkPath.empty() && isSystemModule()) {
     Bits.ModuleDecl.IsNonUserModule = true;
     return;
   }
 
-  // If we loaded a serialized module, check if it was compiler adjacent or in
-  // the SDK.
-
   auto *LF = dyn_cast_or_null<LoadedFile>(newUnit);
   if (!LF)
     return;
 
-  StringRef runtimePath = searchPathOpts.RuntimeResourcePath;
+  // Clang submodules have no source path. We'll check them by retrieving
+  // the top level module on request (though in general, nothing asking for
+  // whether a module is a non-user module should be working on submodules).
   StringRef modulePath = LF->getSourceFilename();
-  if ((!runtimePath.empty() && prefixMatches(runtimePath, modulePath)) ||
+  if (modulePath.empty())
+    return;
+
+  StringRef runtimePath = searchPathOpts.RuntimeResourcePath;
+  if (isStdlibModule() ||
+      (!runtimePath.empty() && prefixMatches(runtimePath, modulePath)) ||
       (!sdkPath.empty() && prefixMatches(sdkPath, modulePath))) {
     Bits.ModuleDecl.IsNonUserModule = true;
   }
@@ -670,7 +688,9 @@ void ModuleDecl::addFile(FileUnit &newFile) {
   Files.push_back(&newFile);
   clearLookupCache();
 
-  updateNonUserModule(&newFile);
+  if (Files.size() == 1) {
+    updateNonUserModule(&newFile);
+  }
 }
 
 void ModuleDecl::addAuxiliaryFile(SourceFile &sourceFile) {