[Bug fortran/79854] diagnostics: gfc_conv_constant_to_tree should be gfc_internal_error

[dominiq at lps dot ens.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79854

--- Comment #4 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
> Unless someone can trigger this error or can prove that gfc_internal_error
> is available when the error ought to trigger, then please leave the
> code as is.

Would not it be better to use

gcc_unreachable ();

as it is done in numerous places?