[Patch, Fortran, OOP] PR 50960: vtables not marked as constant

[Salvatore Filippone]

Well, I must have done something terribly wrong on my first attempt
with -flto, recompilìing and relinking brings situation back to
normal. My bad.
The difference with respect to 4.6.2  in one part of the code is below
measurement noise, in another part is still around 1.3%, barely
perceptible above noise, but still there.

Salvatore