[circt-verilog-lsp-server] No `llvm::StdThreadPool` if LLVM_ENABLE_THREADS is not defined

[teqdruid]

In file included from /workspace/circt/lib/Tools/circt-verilog-lsp-server/LSPServer.cpp:10:
/workspace/circt/lib/Tools/circt-verilog-lsp-server/Utils/PendingChanges.h:107:9: error: no type named 'StdThreadPool' in namespace 'llvm'
  107 |   llvm::StdThreadPool pool;

looking at LLVM's ThreadPool.h:

#if LLVM_ENABLE_THREADS
/// A ThreadPool implementation using std::threads.
///
/// The pool keeps a vector of threads alive, waiting on a condition variable
/// for some work to become available.
class LLVM_ABI StdThreadPool : public ThreadPoolInterface {

So the build fails with -DLLVM_ENABLE_THREADS=OFF.