[clang] Fix UB when string.front() is used for the empty string

[DmitryPolukhin]

Summary

Compilation database might have empty string as a command line argument.
But ExpandResponseFilesDatabase::expand doesn't expect this and assumes
that string.front() can be used for any argument. It is undefined behaviour if
string is empty. With debug build mode it causes crash in clangd.

Test Plan: check-clang

[DmitryPolukhin]

Replace tab with spaces

[DmitryPolukhin]

Fix clang-tidy style warning

clang/lib/Tooling/ExpandResponseFilesCompilationDatabase.cpp

 private:
   std::vector<CompileCommand> expand(std::vector<CompileCommand> Cmds) const {
     for (auto &Cmd : Cmds) {
       bool SeenRSPFile = false;
       llvm::SmallVector<const char *, 20> Argv;
       Argv.reserve(Cmd.CommandLine.size());
       for (auto &Arg : Cmd.CommandLine) {
         Argv.push_back(Arg.c_str());
+        if (!Arg.empty())
           SeenRSPFile |= Arg.front() == '@';
       }
       if (!SeenRSPFile)
         continue;
       llvm::BumpPtrAllocator Alloc;
       llvm::StringSaver Saver(Alloc);
       llvm::cl::ExpandResponseFiles(Saver, Tokenizer, Argv, false, false,
                                     llvm::StringRef(Cmd.Directory), *FS);
       // Don't assign directly, Argv aliases CommandLine.

clang/unittests/Tooling/CompilationDatabaseTest.cpp

 class MemDBTest : public ::testing::Test {
 protected:
   // Adds an entry to the underlying compilation database.
   // A flag is injected: -D <File>, so the command used can be identified.
   void add(StringRef File, StringRef Clang, StringRef Flags) {
     SmallVector<StringRef, 8> Argv = {Clang, File, "-D", File};
     llvm::SplitString(Flags, Argv);
 
+    // Trim double quotation from the argumnets if any.
+    for (auto *It = Argv.begin(); It != Argv.end(); ++It)
+      *It = It->trim("\"");
+
     SmallString<32> Dir;
     llvm::sys::path::system_temp_directory(false, Dir);
 
     Entries[path(File)].push_back(
         {Dir, path(File), {Argv.begin(), Argv.end()}, "foo.o"});
   }
   void add(StringRef File, StringRef Flags = "") { add(File, "clang", Flags); }
 
TEST_F(ExpandResponseFilesTest, ExpandResponseFiles) {
   addFile(path(StringRef("rsp1.rsp")), "-Dflag");
 
   add("foo.cpp", "clang", "@rsp1.rsp");
   add("bar.cpp", "clang", "-Dflag");
   EXPECT_EQ(getCommand("foo.cpp"), "clang foo.cpp -D foo.cpp -Dflag");
   EXPECT_EQ(getCommand("bar.cpp"), "clang bar.cpp -D bar.cpp -Dflag");
 }
 
+TEST_F(ExpandResponseFilesTest, ExpandResponseFilesEmptyArgument) {
+  addFile(path(StringRef("rsp1.rsp")), "-Dflag");
+
+  add("foo.cpp", "clang", "@rsp1.rsp \"\"");
+  EXPECT_EQ(getCommand("foo.cpp"), "clang foo.cpp -D foo.cpp -Dflag ");
+}
+
 } // end namespace tooling
 } // end namespace clang