[Bug c++/85052] Implement support for clang's __builtin_convertvector

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85052

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Note, I've posted in the meantime a newer version of the patch that should
handle the 2x narrowing or 2x widening cases better, see
https://gcc.gnu.org/ml/gcc-patches/2019-01/msg00129.html