mlir/{SPIRV,Bufferization}: use std::optional in .td files (NFC)

mlir/{SPIRV,Bufferization}: use std::optional in .td files (NFC)

This is part of an effort to migrate from llvm::Optional to
std::optional. 22426110c5ef changed the way mlir-tblgen generates .inc
files, emitting std::optional when an Optional attribute is specified in
a .td file. It also changed several .td files hard-coding llvm::Optional
to use std::optional. However, the patch excluded a few .td files in
SPIRV and Bufferization hard-coding llvm::Optional. This patch fixes
that defect, and after this patch, references to llvm::Optional in .cpp
and .h files can be replaced mechanically.

See also: https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

Signed-off-by: Ramkumar Ramachandra <r@artagnon.com>
Differential Revision: https://reviews.llvm.org/D140329