[mlir][GPU] Allow specifying alignment of memory attributions

Add support for argument attributes on workgroup and private
attributions for GPU functions. These arguments are outside the range
of getNumArguments() and get printed separately, so the default
mechanism for function argument attributes can't be used on them.

Having done this, check for the `llvm.align` attribute on workgroup or
private attributions in a `gpu.func` and pass it through to the
relevant allocation op (creating a global or alloca). This allows
people creating kernels that use multiple workgroup buffers to set an
alignment.

(This could, in the future, be a GPU dialect `alignment` attribute,
but I've taken the simpler route of using the LLVM version instead for
simplicity and because I don't know how this might impact backends
like Vulkan)

Reviewed By: nirvedhmeshram

Differential Revision: https://reviews.llvm.org/D148965