[SYCL] Make sub_group non-user-constructible in preview mode

[KornevNikita]

SYCL 2020 section 4.9.1.8 specifies sycl::sub_group with a deleted default
constructor, but the implementation declared it protected and defaulted
instead. That is not equivalent: a constructor that is defaulted on its first
declaration is not user-provided, so in C++17 sub_group remained an
aggregate and sycl::sub_group{} was a well-formed aggregate initialization
that bypassed the access check altogether:

// Compiles today with -std=c++17, which is the default:
auto sg = sycl::sub_group{};

This patch deletes the default constructor and adds a protected one taking a
private tag. Being user-provided, it also makes the class a non-aggregate,
which closes the C++17 loophole. sub_group objects are now created through
that constructor in nd_item::get_sub_group() and
ext::oneapi::this_work_item::get_sub_group(), both of which are already
friends.

Applications may rely on the current permissive behavior, so the change is
guarded by __INTEL_PREVIEW_BREAKING_CHANGES.

The deprecated ext::oneapi::sub_group also stops initializing its base
through a private default constructor: copy-initializing the base works in
both modes and needs no guard.

Testing

New sycl/test/basic_tests/sub_group_interface.cpp checks the SYCL 2020
synopsis, the common by-value semantics, and that the class can be constructed
neither directly nor as an aggregate. It runs in four configurations:
{C++17, C++20} x {default, -fpreview-breaking-changes}.

Verified that legitimate ways of obtaining a sub-group still compile in all
four configurations (nd_item::get_sub_group(), the free function queries,
khr::this_sub_group(), copy/assign/compare, implicit conversion to the
deprecated ext::oneapi::sub_group, group algorithms), and that
sycl::sub_group{} still compiles outside of preview mode.

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