[SYCL 2020] Add accessor simplifications/deduction guides, host_accessor

[illuhad]

(cc @tomdeakin)

Initial implementation of

• Accessor simplifications, deduction guides and no_init property
• host_accessor (partial implementation)
• Redesigned property_list which was required by the new accessor interface.

Allows code patterns such as

q.submit([&](sycl::handler& cgh){
  // read-write default for non-const buffer type
  sycl::accessor acc{buff, cgh};
  // Using deduction guides and no_init property
  sycl::accessor acc2{buff, cgh, sycl::read_only, sycl::no_init};
});
sycl::host_accessor hacc{buff, sycl::read_only};

Note: This PR does not yet contain additional changes/additions to the members of accessors, such as the iterator interface.

The only thing I'm not sure about is the pattern:

sycl::buffer<int> buff {...};
// construct read-only accessor
sycl::accessor<const int> acc{buff};

I think originally the intent was to allow this pattern in order to make read-write/read-only expressible only with accessor const-ness, but I don't see how the interface in the spec allows for that, and the wording in the spec appears pretty vague to me.
The spec hardcodes that the buffer arguments to the accessor<T> constructors is of type buffer<T>.

We would need additional accessor(buffer<std::remove_const_t<T>>&) constructors to make constructing accessor<const T> from buffer<T> work, which the spec does not seem to anticipate (which would also double the number of constructors because we don't have enough yet ;-) )

[illuhad]

@hipSYCL-ci-bot test again