[llvm] [Support][Cygwin] Fix handling of Process symbol lookup. (PR #143072)

[Tomohiro Kashiwada via llvm-commits]

kikairoya wrote:

`llvm::DynamicLibrary` already has a symbol representing invalid state. How about to use this?

https://github.com/llvm/llvm-project/blob/95b3fd61708ca5472df05b478c291c8d16dc37d5/llvm/include/llvm/Support/DynamicLibrary.h#L38

https://github.com/llvm/llvm-project/pull/143072