[libcxx] Throw correct exception from std::vector::reserve

According to the standard [vector.capacity]/5, std::vector<T>::reserve
shall throw an exception of type std::length_error when the requested
capacity exceeds max_size().

This behavior is not implemented correctly: the function 'reserve'
simply propagates the exception from allocator<T>::allocate. Before
D110846 that exception used to be of type std::length_error (which is
correct for vector<T>::reserve, but incorrect for
allocator<T>::allocate).

This patch fixes the issue and adds regression tests.

Reviewed By: Quuxplusone, ldionne, #libc

Differential Revision: https://reviews.llvm.org/D112068