[lldb] Minor cleanups at callsites of FileSpec::GetFileNameExtension

[bulbazord]

Summary

FileSpec::GetFileNameExtension returns a StringRef. In some cases we
are calling it and then storing the result in a local. To prevent
cases where we store the StringRef, mutate the Filespec, and then try to
use the stored StringRef afterwards, I've audited the callsites and made
adjustments to mitigate: Either marking the FileSpec it comes from as
const (to avoid mutations) or by not storing the StringRef in a local if
it makes sense not to.

lldb/source/Commands/CommandObjectTarget.cpp

protected:
   bool DoExecute(Args &command, CommandReturnObject &result) override {
     if (command.GetArgumentCount() != 1) {
       result.AppendErrorWithFormat("'%s' takes exactly one pcm path argument.",
                                    m_cmd_name.c_str());
       return false;
     }
 
     const char *pcm_path = command.GetArgumentAtIndex(0);
-    FileSpec pcm_file{pcm_path};
+    const FileSpec pcm_file{pcm_path};
 
     if (pcm_file.GetFileNameExtension() != ".pcm") {
       result.AppendError("file must have a .pcm extension");
       return false;
     }
 
     if (!FileSystem::Instance().Exists(pcm_file)) {
       result.AppendError("pcm file does not exist");

lldb/source/Plugins/Platform/MacOSX/PlatformDarwinKernel.h

protected:
   // Returns true if there is a .dSYM bundle with NO kernel binary next to it
   static bool
   KerneldSYMHasNoSiblingBinary(const FileSpec &kernel_dsym_filepath);
 
   // Given a dsym_bundle argument ('.../foo.dSYM'), return a FileSpec
   // with the binary inside it ('.../foo.dSYM/Contents/Resources/DWARF/foo').
   // A dSYM bundle may have multiple DWARF binaries in them, so a vector
   // of matches is returned.
-  static std::vector<FileSpec> GetDWARFBinaryInDSYMBundle(FileSpec dsym_bundle);
+  static std::vector<FileSpec>
+  GetDWARFBinaryInDSYMBundle(const FileSpec &dsym_bundle);
 
   Status GetSharedModuleKext(const ModuleSpec &module_spec, Process *process,
                              lldb::ModuleSP &module_sp,
                              const FileSpecList *module_search_paths_ptr,
                              llvm::SmallVectorImpl<lldb::ModuleSP> *old_modules,
                              bool *did_create_ptr);
 
   Status GetSharedModuleKernel(

lldb/source/Plugins/Platform/MacOSX/PlatformDarwinKernel.cpp

 // Helper function to find *.sdk and *.kdk directories in a given directory.
 FileSystem::EnumerateDirectoryResult
 PlatformDarwinKernel::FindKDKandSDKDirectoriesInDirectory(
     void *baton, llvm::sys::fs::file_type ft, llvm::StringRef path) {
   static constexpr llvm::StringLiteral g_sdk_suffix = ".sdk";
   static constexpr llvm::StringLiteral g_kdk_suffix = ".kdk";
 
   PlatformDarwinKernel *thisp = (PlatformDarwinKernel *)baton;
-  FileSpec file_spec(path);
+  const FileSpec file_spec(path);
   if (ft == llvm::sys::fs::file_type::directory_file &&
       (file_spec.GetFileNameExtension() == g_sdk_suffix ||
        file_spec.GetFileNameExtension() == g_kdk_suffix)) {
     AddRootSubdirsToSearchPaths(thisp, file_spec.GetPath());
   }
   return FileSystem::eEnumerateDirectoryResultNext;
 }
 

 
 FileSystem::EnumerateDirectoryResult
 PlatformDarwinKernel::GetKernelsAndKextsInDirectoryHelper(
     void *baton, llvm::sys::fs::file_type ft, llvm::StringRef path,
     bool recurse) {
   static constexpr llvm::StringLiteral g_kext_suffix = ".kext";
   static constexpr llvm::StringLiteral g_dsym_suffix = ".dSYM";
 
-  FileSpec file_spec(path);
+  const FileSpec file_spec(path);
   llvm::StringRef file_spec_extension = file_spec.GetFileNameExtension();
 
   Log *log = GetLog(LLDBLog::Platform);
 
   LLDB_LOGV(log, "PlatformDarwinKernel examining '{0}'", file_spec);
 
   PlatformDarwinKernel *thisp = (PlatformDarwinKernel *)baton;
 

 }
 
 // TODO: This method returns a vector of FileSpec's because a
 // dSYM bundle may contain multiple DWARF binaries, but it
 // only implements returning the base name binary for now;
 // it should iterate over every binary in the DWARF subdir
 // and return them all.
 std::vector<FileSpec>
-PlatformDarwinKernel::GetDWARFBinaryInDSYMBundle(FileSpec dsym_bundle) {
+PlatformDarwinKernel::GetDWARFBinaryInDSYMBundle(const FileSpec &dsym_bundle) {
   std::vector<FileSpec> results;
   static constexpr llvm::StringLiteral g_dsym_suffix = ".dSYM";
   if (dsym_bundle.GetFileNameExtension() != g_dsym_suffix) {
     return results;
   }
   // Drop the '.dSYM' from the filename
   std::string filename =
       dsym_bundle.GetFileNameStrippingExtension().GetCString();

lldb/source/Plugins/ScriptInterpreter/Lua/Lua.cpp

llvm::Error e = llvm::make_error<llvm::StringError>(
       llvm::formatv("{0}\n", lua_tostring(m_lua_state, -1)),
       llvm::inconvertibleErrorCode());
   // Pop error message from the stack.
   lua_pop(m_lua_state, 1);
   return e;
 }
 
 llvm::Error Lua::LoadModule(llvm::StringRef filename) {
-  FileSpec file(filename);
+  const FileSpec file(filename);
   if (!FileSystem::Instance().Exists(file)) {
     return llvm::make_error<llvm::StringError>("invalid path",
                                                llvm::inconvertibleErrorCode());
   }
 
-  llvm::StringRef module_extension = file.GetFileNameExtension();
-  if (module_extension != ".lua") {
+  if (file.GetFileNameExtension() != ".lua") {
     return llvm::make_error<llvm::StringError>("invalid extension",
                                                llvm::inconvertibleErrorCode());
   }
 
   int error = luaL_loadfile(m_lua_state, filename.data()) ||
               lua_pcall(m_lua_state, 0, 1, 0);
   if (error != LUA_OK) {
     llvm::Error e = llvm::make_error<llvm::StringError>(