libstdc++: Implement LWG 2762 for std::unique_ptr::operator*

The LWG issue proposes to add a conditional noexcept-specifier to
std::unique_ptr's dereference operator. The issue is currently in
Tentatively Ready status, but even if it isn't voted into the draft, we
can do it as a conforming extensions. This commit also adds a similar
noexcept-specifier to operator[] for the unique_ptr<T[], D> partial
specialization.

Also ensure that all dereference operators for shared_ptr are noexcept,
and adds tests for the std::optional accessors modified by the issue,
which were already noexcept in our implementation.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr_base.h (__shared_ptr_access::operator[]):
	Add noexcept.
	* include/bits/unique_ptr.h (unique_ptr::operator*): Add
	conditional noexcept as per LWG 2762.
	* testsuite/20_util/shared_ptr/observers/array.cc: Check that
	dereferencing cannot throw.
	* testsuite/20_util/shared_ptr/observers/get.cc: Likewise.
	* testsuite/20_util/optional/observers/lwg2762.cc: New test.
	* testsuite/20_util/unique_ptr/lwg2762.cc: New test.

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