Problem: Check that mapping rhs starts with lhs doesn't work if lhs is
not simplified.
Solution: Keep track of the mapblock containing the alternative lhs and
also compare with it (zeertzjq).
fixes: vim/vim#15376closes: vim/vim#153849d997addc7
Cherry-pick removal of save_m_str from patch 8.2.4059.
Remove `export` pramgas from defs headers as it causes IWYU to believe
that the definitions from the defs headers comes from main header, which
is not what we really want.
