[Bug libstdc++/86655] std::assoc_legendre should not constrain the value of m

[kretz at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86655

--- Comment #2 from Matthias Kretz <kretz at kde dot org> ---
http://eel.is/c++draft/c.math#sf.cmath-1.3 might be the reason why `m <= l` is
enforced. But unless I'm confused the footnote on "mathematically defined"
tells us it should work:

- "(a) if it is explicitly defined for that set of argument values" - does not
hold

- "(b) if its limiting value exists and does not depend on the direction of
approach" - this holds, no?