[clangd] Parse files without extensions if we don't have a compile command.

[hokein]

Summary

This would enable clangd for C++ standard library files.

clang-tools-extra/clangd/GlobalCompilationDatabase.cpp

static std::string getFallbackClangPath() {
   ClangPath = llvm::sys::path::parent_path(ClangdExecutable);
   llvm::sys::path::append(ClangPath, "clang");
   return ClangPath.str();
 }
 
 tooling::CompileCommand
 GlobalCompilationDatabase::getFallbackCommand(PathRef File) const {
   std::vector<std::string> Argv = {getFallbackClangPath()};
-  // Clang treats .h files as C by default, resulting in unhelpful diagnostics.
+  // Clang treats .h files as C by default and files without extension as linker
+  // input, resulting in unhelpful diagnostics.
   // Parsing as Objective C++ is friendly to more cases.
-  if (llvm::sys::path::extension(File) == ".h")
+  auto FileExtension = llvm::sys::path::extension(File);
+  if (FileExtension.empty() || FileExtension == ".h")
     Argv.push_back("-xobjective-c++-header");
   Argv.push_back(File);
   tooling::CompileCommand Cmd(llvm::sys::path::parent_path(File),
                               llvm::sys::path::filename(File), std::move(Argv),
                               /*Output=*/"");
   Cmd.Heuristic = "clangd fallback";
   return Cmd;
 }

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

EXPECT_THAT(Cmd.CommandLine,
               ElementsAre(EndsWith("clang"), testPath("foo/bar.cc")));
   EXPECT_EQ(Cmd.Output, "");
 
   // .h files have unknown language, so they are parsed liberally as obj-c++.
   Cmd = DB.getFallbackCommand(testPath("foo/bar.h"));
   EXPECT_THAT(Cmd.CommandLine,
               ElementsAre(EndsWith("clang"), "-xobjective-c++-header",
                           testPath("foo/bar.h")));
+  Cmd = DB.getFallbackCommand(testPath("foo/bar"));
+  EXPECT_THAT(Cmd.CommandLine,
+              ElementsAre(EndsWith("clang"), "-xobjective-c++-header",
+                          testPath("foo/bar")));
 }
 
 static tooling::CompileCommand cmd(llvm::StringRef File, llvm::StringRef Arg) {
   return tooling::CompileCommand(testRoot(), File, {"clang", Arg, File}, "");
 }
 
 class OverlayCDBTest : public ::testing::Test {
   class BaseCDB : public GlobalCompilationDatabase {