Prepopulate the CompilerInvocation with the host plugin search path.

Generally, LLDB takes compiler plugin search paths from serialized
options in Swift modules, however, for the REPL and Playgrounds it is
useful to always add the host toolchain plugins by default.

Unfortunately this is a property of the toolchain lldb is installed
with, so this is not really testable from within an uninstalled build.

rdar://112122752

Author: adrian-prantl

lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp

@@ -1032,10 +1032,10 @@ SwiftASTContextForModule::~SwiftASTContextForModule() {
 
 /// This code comes from CompilerInvocation.cpp (setRuntimeResourcePath).
 static void ConfigureResourceDirs(swift::CompilerInvocation &invocation,
-                                  FileSpec resource_dir, llvm::Triple triple) {
+                                  StringRef resource_dir, llvm::Triple triple) {
   // Make sure the triple is right:
   invocation.setTargetTriple(triple.str());
-  invocation.setRuntimeResourcePath(resource_dir.GetPath().c_str());
+  invocation.setRuntimeResourcePath(resource_dir);
 }
 
 static const char *getImportFailureString(swift::serialization::Status status) {
@@ -1930,8 +1930,8 @@ SwiftASTContext::CreateInstance(lldb::LanguageType language, Module &module,
 
   std::string resource_dir =
       HostInfo::GetSwiftResourceDir(triple, swift_ast_sp->GetPlatformSDKPath());
-  ConfigureResourceDirs(swift_ast_sp->GetCompilerInvocation(),
-                        FileSpec(resource_dir), triple);
+  ConfigureResourceDirs(swift_ast_sp->GetCompilerInvocation(), resource_dir,
+                        triple);
 
   // Apply the working directory to all relative paths.
   std::vector<std::string> DeserializedArgs = swift_ast_sp->GetClangArguments();
@@ -2390,8 +2390,8 @@ lldb::TypeSystemSP SwiftASTContext::CreateInstance(
   llvm::Triple triple = swift_ast_sp->GetTriple();
   std::string resource_dir = HostInfo::GetSwiftResourceDir(
       triple, swift_ast_sp->GetPlatformSDKPath());
-  ConfigureResourceDirs(swift_ast_sp->GetCompilerInvocation(),
-                        FileSpec(resource_dir), triple);
+  ConfigureResourceDirs(swift_ast_sp->GetCompilerInvocation(), resource_dir,
+                        triple);
   const bool discover_implicit_search_paths =
       target.GetSwiftDiscoverImplicitSearchPaths();
 
@@ -2813,8 +2813,7 @@ void SwiftASTContext::InitializeSearchPathOptions(
   llvm::Triple triple(GetTriple());
   std::string resource_dir =
       HostInfo::GetSwiftResourceDir(triple, GetPlatformSDKPath());
-  ConfigureResourceDirs(GetCompilerInvocation(), FileSpec(resource_dir),
-                        triple);
+  ConfigureResourceDirs(GetCompilerInvocation(), resource_dir, triple);
 
   std::string sdk_path = GetPlatformSDKPath().str();
   if (TargetSP target_sp = GetTargetWP().lock())
@@ -2855,6 +2854,20 @@ void SwiftASTContext::InitializeSearchPathOptions(
     lpaths.insert(lpaths.begin(), "/usr/lib/swift");
   }
 
+  // Set the default host plugin paths.
+  llvm::SmallString<256> plugin_path;
+  llvm::sys::path::append(plugin_path, resource_dir, "host", "plugins");
+  if (!FileSystem::Instance().Exists(plugin_path)) {
+    LOG_PRINTF(GetLog(LLDBLog::Types), "Host plugin path %s does not exist",
+               plugin_path.str().str().c_str());
+  } else {
+    std::string server = SwiftASTContext::GetPluginServer(plugin_path);
+    if (!server.empty() && FileSystem::Instance().Exists(server))
+      invocation.getSearchPathOptions().PluginSearchOpts.emplace_back(
+          swift::PluginSearchOption::ExternalPluginPath{plugin_path.str().str(),
+                                                        server});
+  }
+
   llvm::StringMap<bool> processed;
   std::vector<std::string> invocation_import_paths(
       invocation.getSearchPathOptions().getImportSearchPaths());