llvm: Disable copy for SingleThreadExecutor (#168782)
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
