libstdc++: Remove unnecessary tag dispatching in std::vector
There's no need to call a _M_xxx_dispatch function with a
statically-known __false_type tag, we can just directly call the
function that should be dispatched to. This will compile a tiny bit
faster and save a function call with optimization or inlining turned
off.
Also add the always_inline attribute to the __iterator_category helper
used for dispatching on the iterator category.