alternative to #24101#23892 changed the opensym experimental switch so that it has to be
enabled in the context of the generic/template declarations capturing
the symbols, not the context of the instantiation of the
generics/templates. This was to be in line with where the compiler gives
the warnings and changes behavior in a potentially breaking way.
However `results` [depends on the old
behavior](71d404b314/results.nim (L1428)),
so that the callers of the macros provided by results always take
advantage of the opensym behavior. To accomodate this, we change the
behavior of the old experimental option that `results` uses,
`genericsOpenSym`, so that ignores the information of whether or not
symbols are intentionally opened and always gives the opensym behavior
as long as it's enabled at instantiation time. This should keep
`results` working as is. However this differs from the normal opensym
switch in that it doesn't generate `nnkOpenSym`.
Before it was just a generics-only version of `openSym` along with
`templateOpenSym` which was only for templates. So `templateOpenSym` is
removed along with this change, but no one appears to have used it.
refs https://github.com/nim-lang/Nim/pull/23873#discussion_r1687995060,
fixes#23386, fixes#23385, supersedes #23572
Turns the `nfOpenSym` node flag implemented in #23091 and extended in
#23102 and #23873, into a node kind `nkOpenSym` that forms a unary node
containing either `nkSym` or `nkOpenSymChoice`. Since this affects
macros working on generic proc AST, the node kind is now only generated
when the experimental switch `genericsOpenSym` is enabled, and a new
node flag `nfDisabledOpenSym` is set to the `nkSym` or `nkOpenSymChoice`
when the switch is not enabled so that we can give a warning.
Now that the experimental switch has more reasonable semantics, we
define `nimHasGenericsOpenSym2`.
