llvm: Disable copy for SingleThreadExecutor

This is a workaround for the MSVC compiler, which attempts to generate a
copy assignment operator implementation for classes marked as
`__declspec(dllexport)`. Explicitly marking the copy assignment operator
as deleted works around the problem.

DevCom ticket: https://developercommunity.microsoft.com/t/Classes-marked-with-__declspecdllexport/11003192

Author: Steelskin

llvm/include/llvm/Support/ThreadPool.h

@@ -225,6 +225,13 @@ class LLVM_ABI SingleThreadExecutor : public ThreadPoolInterface {
   /// Blocking destructor: the pool will first execute the pending tasks.
   ~SingleThreadExecutor() override;
 
+  // Excplicitly disable copy. This is necessary for the MSVC LLVM_DYLIB build
+  // to work around a compiler bug where classes marked as
+  // `__declspec(dllexport)` cause the compiler to attempt to generate a copy
+  // assignment operator.
+  SingleThreadExecutor(const SingleThreadExecutor &) = delete;
+  SingleThreadExecutor &operator=(const SingleThreadExecutor &) = delete;
+
   /// Blocking wait for all the tasks to execute first
   void wait() override;