When building with latest MSVC on Windows, this fixes some compile-time
warnings from last week's integration in
https://github.com/llvm/llvm-project/pull/157885:
```
[321/5941] Building CXX object lib\Support\LSP\CMakeFiles\LLVMSupportLSP.dir\Transport.cpp.obj
C:\git\llvm-project\llvm\lib\Support\LSP\Transport.cpp(123): warning C4930: 'std::lock_guard<std::mutex> responseHandlersLock(llvm::lsp::MessageHandler::ResponseHandlerTy)': prototyped function not called (was a variable definition intended?)
[384/5941] Building CXX object unittests\Support\LSP\CMakeFiles\LLVMSupportLSPTests.dir\Transport.cpp.obj
C:\git\llvm-project\llvm\unittests\Support\LSP\Transport.cpp(190): warning C4804: '+=': unsafe use of type 'bool' in operation
```
da55134db3