[Tooling][JSONCompilationDatabase] Fix backslash escaping on Windows

[lh123]

Summary

We should always use TokenizeWindowsCommandLine on Windows.

When compiling clangd with MinGW-W64, Triple.getEnvironment will return EnvironmentType::GNU.

fixes https://github.com/clangd/clangd/issues/92.

[lh123]

clang/lib/Tooling/JSONCompilationDatabase.cpp

153

I think we can use TokenizeGNUCommandLine instead of CommandLineArgumentParser.

clang/lib/Tooling/JSONCompilationDatabase.cpp

 };
 
 std::vector<std::string> unescapeCommandLine(JSONCommandLineSyntax Syntax,
                                              StringRef EscapedCommandLine) {
   if (Syntax == JSONCommandLineSyntax::AutoDetect) {
     Syntax = JSONCommandLineSyntax::Gnu;
     llvm::Triple Triple(llvm::sys::getProcessTriple());
     if (Triple.getOS() == llvm::Triple::OSType::Win32) {
-      // Assume Windows command line parsing on Win32 unless the triple
-      // explicitly tells us otherwise.
-      if (!Triple.hasEnvironment() ||
-          Triple.getEnvironment() == llvm::Triple::EnvironmentType::MSVC)
-        Syntax = JSONCommandLineSyntax::Windows;
     }
   }
 
   if (Syntax == JSONCommandLineSyntax::Windows) {
     llvm::BumpPtrAllocator Alloc;
     llvm::StringSaver Saver(Alloc);
     llvm::SmallVector<const char *, 64> T;
     llvm::cl::TokenizeWindowsCommandLine(EscapedCommandLine, Saver, T);
     std::vector<std::string> Result(T.begin(), T.end());
     return Result;
   }
   assert(Syntax == JSONCommandLineSyntax::Gnu);
   CommandLineArgumentParser parser(EscapedCommandLine);
   return parser.parse();
 }
 
 // This plugin locates a nearby compile_command.json file, and also infers
 // compile commands for files not present in the database.
 class JSONCompilationDatabasePlugin : public CompilationDatabasePlugin {
   std::unique_ptr<CompilationDatabase>
   loadFromDirectory(StringRef Directory, std::string &ErrorMessage) override {