libstdc++: Negative tests for constexpr uses inplace_vector [PR119137]

libstdc++: Negative tests for constexpr uses inplace_vector [PR119137]

Adds negative tests for preconditions on inserting into a full
inplace_vector and erasing non-existent elementsi at compile-time.
This ensures coverage for the inplace_vector<T, 0> specialization.

Also extends element access tests to cover front() and back()
methods, and const and mutable overloads for all accesses.

	PR libstdc++/119137

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/inplace_vector/access/elem.cc: Cover
	front and back methods and const calls.
	* testsuite/23_containers/inplace_vector/access/elem_neg.cc:
	Likewise.
	* testsuite/23_containers/inplace_vector/modifiers/erase_neg.cc:
	New test.
	* testsuite/23_containers/inplace_vector/modifiers/single_insert_neg.cc:
	New test.