[clangd] Set correct CWD when using compile_flags.txt

[adamcz]

Summary

This fixes a bug where clangd would attempt to set CWD to the
compile_flags.txt file itself.

[sammccall]

Yikes, thanks!

clang-tools-extra/clangd/GlobalCompilationDatabase.cpp

if (auto CDB = tooling::JSONCompilationDatabase::loadFromBuffer(
     return tooling::inferTargetAndDriverMode(
         tooling::inferMissingCompileCommands(
             expandResponseFiles(std::move(CDB), std::move(FS))));
   }
   return nullptr;
 }
 static std::unique_ptr<tooling::CompilationDatabase>
 parseFixed(PathRef Path, llvm::StringRef Data, std::string &Error) {
-  return tooling::FixedCompilationDatabase::loadFromBuffer(Path, Data, Error);
+  return tooling::FixedCompilationDatabase::loadFromBuffer(
+      llvm::sys::path::parent_path(Path), Data, Error);
 }
 
 bool DirectoryBasedGlobalCompilationDatabase::DirectoryCache::load(
     llvm::vfs::FileSystem &FS) {
   dlog("Probing directory {0}", Path);
   std::string Error;
 
   // Load from the specially-supported compilation databases (JSON + Fixed).

clang-tools-extra/clangd/unittests/GlobalCompilationDatabaseTests.cpp

const char *const CDB =
       )cdb";
   FS.Files[testPath("x/build/compile_commands.json")] =
       llvm::formatv(CDB, llvm::sys::path::convert_to_slash(testRoot()));
   EXPECT_THAT(Command("x/foo.cc"), Contains("-DXYZZY"));
   EXPECT_THAT(Command("bar.cc"), IsEmpty())
       << "x/build/compile_flags.json only applicable to x/";
 }
 
+TEST(GlobalCompilationDatabaseTest, CompileFlagsDirectory) {
+  MockFS FS;
+  FS.Files[testPath("x/compile_flags.txt")] = "-DFOO";
+  DirectoryBasedGlobalCompilationDatabase CDB(FS);
+  auto Commands = CDB.getCompileCommand(testPath("x/y.cpp"));
+  ASSERT_TRUE(Commands.hasValue());
+  EXPECT_THAT(Commands.getValue().CommandLine, Contains("-DFOO"));
+  // Make sure we pick the right working directory.
+  EXPECT_EQ(testPath("x"), Commands.getValue().Directory);
+}
+
 TEST(GlobalCompilationDatabaseTest, NonCanonicalFilenames) {
   OverlayCDB DB(nullptr);
   std::vector<std::string> DiscoveredFiles;
   auto Sub =
       DB.watch([&DiscoveredFiles](const std::vector<std::string> Changes) {
         DiscoveredFiles = Changes;
       });