Assignment with colons *should* be tilde expanded in POSIX mode

[Anders Kaseorg]

As you know, POSIX requires tilde expansion following an an unquoted colon in 
an assignment [1]. A bug was reported [2] against bash 5.1-alpha that the 
tildes in

    $ echo foo=~:~
    foo=~:~

should not be expanded in POSIX mode, because this is not an assignment. That 
was fixed in 5.1-beta. However, that fix also seems to have broken the actual 
assignment

    $ foo=~:~
    $ echo "$foo"
    /home/anders:~

where both tildes should be expanded in POSIX mode, because this is an 
assignment.

5.0.18 and 5.1-alpha gave the expected /home/anders:/home/anders, while 
5.1-beta, 5.1.8, and the devel branch incorrectly give /home/anders:~.

Anders

[1] 
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_01
[2] https://lists.gnu.org/archive/html/bug-bash/2020-07/msg00072.html