[clang-tidy] Fail gracefully upon empty database fields

[serge-sans-paille]

Summary

fix https://bugs.llvm.org/show_bug.cgi?id=42281

[jkorous]

LGTM

[serge-sans-paille]

Thanks @jkorous for the fast review o/

clang-tools-extra/test/clang-tidy/empty-database.cpp

+// RUN: not clang-tidy -p %S/empty-database %s 2>&1 | FileCheck %s
+
+// CHECK: LLVM ERROR: Cannot chdir into ""!

clang-tools-extra/test/clang-tidy/empty-database/compile_commands.json

+[{
+    "directory":"",
+    "file":"/tmp/","arguments":[]
+}]

clang/lib/Tooling/InterpolatingCompilationDatabase.cpp

llvm::opt::InputArgList ArgList;
       ArgList = {TmpArgv.begin(), TmpArgv.end()};
     }
 
     // Parse the old args in order to strip out and record unwanted flags.
     // We parse each argument individually so that we can retain the exact
     // spelling of each argument; re-rendering is lossy for aliased flags.
     // E.g. in CL mode, /W4 maps to -Wall.
     auto OptTable = clang::driver::createDriverOptTable();
+    if(!OldArgs.empty())
       Cmd.CommandLine.emplace_back(OldArgs.front());
     for (unsigned Pos = 1; Pos < OldArgs.size();) {
       using namespace driver::options;
 
       const unsigned OldPos = Pos;
       std::unique_ptr<llvm::opt::Arg> Arg(OptTable->ParseOneArg(
           ArgList, Pos,
           /* Include */ClangCLMode ? CoreOption | CLOption : 0,
           /* Exclude */ClangCLMode ? 0 : CLOption));
private:
   static bool checkIsCLMode(ArrayRef<std::string> CmdLine) {
     // First look for --driver-mode.
     for (StringRef S : llvm::reverse(CmdLine)) {
       if (S.consume_front("--driver-mode="))
         return S == "cl";
     }
 
     // Otherwise just check the clang executable file name.
-    return llvm::sys::path::stem(CmdLine.front()).endswith_lower("cl");
+    return !CmdLine.empty() && llvm::sys::path::stem(CmdLine.front()).endswith_lower("cl");
   }
 
   // Map the language from the --std flag to that of the -x flag.
   static types::ID toType(InputKind::Language Lang) {
     switch (Lang) {
     case InputKind::C:
       return types::TY_C;
     case InputKind::CXX:

clang/lib/Tooling/Tooling.cpp

for (CompileCommand &CompileCommand : CompileCommandsForFile) {
       // guaranteeing that all subsequent relative path operations work
       // on the same path the original chdir resulted in. This makes a
       // difference for example on network filesystems, where symlinks might be
       // switched during runtime of the tool. Fixing this depends on having a
       // file system abstraction that allows openat() style interactions.
       if (OverlayFileSystem->setCurrentWorkingDirectory(
               CompileCommand.Directory))
         llvm::report_fatal_error("Cannot chdir into \"" +
-                                 Twine(CompileCommand.Directory) + "\n!");
+                                 Twine(CompileCommand.Directory) + "\"!");
 
       // Now fill the in-memory VFS with the relative file mappings so it will
       // have the correct relative paths. We never remove mappings but that
       // should be fine.
       if (SeenWorkingDirectories.insert(CompileCommand.Directory).second)
         for (const auto &MappedFile : MappedFileContents)
           if (!llvm::sys::path::is_absolute(MappedFile.first))
             InMemoryFileSystem->addFile(