This patch replaces LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]],
introduced as part of C++17.

See https://github.com/llvm/llvm-project/pull/147168 for more info.

llvm::is_contained is shorter than llvm::all_of plus a lambda.

As part of the work on transitioning bufferization dialect, ops, and
associated logic to operate on newly added type interfaces (see
00eaff3e9c897c263a879416d0f151d7ca7eeaff), rename the
bufferization.to_memref to highlight the generic nature of the op.

Bufferization process produces buffers while memref is a builtin type
rather than a generic term.

Preserve the current API (to_buffer still produces a memref), however,
as the new type interfaces are not used yet.

Note that PointerUnion::{is,get} have been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

I'm not touching PointerUnion::dyn_cast for now because it's a bit
complicated; we could blindly migrate it to dyn_cast_if_present, but
we should probably use dyn_cast when the operand is known to be
non-null.