libstdc++: Optimize std::to_array for trivial types [PR110167]

libstdc++: Optimize std::to_array for trivial types [PR110167]

As reported in PR libstdc++/110167, std::to_array compiles extremely
slowly for very large arrays. It needs to instantiate a very large
specialization of std::index_sequence<N...> and then create a very large
aggregate initializer from the pack expansion. For trivial types we can
simply default-initialize the std::array and then use memcpy to copy the
values. For non-trivial types we need to use the existing
implementation, despite the compilation cost.

As also noted in the PR, using a generic lambda instead of the
__to_array helper compiles faster since gcc-13. It also produces
slightly smaller code at -O1, due to additional inlining. The code at
-Os, -O2 and -O3 seems to be the same. This new implementation requires
__cpp_generic_lambdas >= 201707L (i.e. P0428R2) but that is supported
since Clang 10 and since Intel icc 2021.5.0 (and since GCC 10.1).

libstdc++-v3/ChangeLog:

	PR libstdc++/110167
	* include/std/array (to_array): Initialize arrays of trivial
	types using memcpy. For non-trivial types, use lambda
	expressions instead of a separate helper function.
	(__to_array): Remove.
	* testsuite/23_containers/array/creation/110167.cc: New test.