[C++20] [P1825] Fix bugs with implicit-move from variables of reference type.

Review D88220 turns out to have some pretty severe bugs, but I *think*
this patch fixes them.

Paper P1825 is supposed to enable implicit move from "non-volatile objects
and rvalue references to non-volatile object types." Instead, what was committed
seems to have enabled implicit move from "non-volatile things of all kinds,
except that if they're rvalue references then they must also refer to non-volatile
things." In other words, D88220 accidentally enabled implicit move from
lvalue object references (super yikes!) and also from non-object references
(such as references to functions).

These two cases are now fixed and regression-tested.

Differential Revision: https://reviews.llvm.org/D98971