[LLVM][ADT] Put both vesions of 'unique' into STLExtras.h (#82312)

Currently there are two versions of llvm::unique, one that requires a
predicate, and is in STLExtras.h; and one that does not require a
predicate, and is in GenericUniformityImpl.h. This moves the one from
GenericUniformityImp.h to STlExtras.h, so they are both together, and
can both be easily called from other places inside LLVM.