fix ast ternary string extension

[Khrig]

#4079

[jix]

As discussed during the Dev JF, there are some subtleties with strings in SV as it defines a dynamically sized string type (6.16) as well as implicitly zero pads string literals in a non string context (11.10), it's not obvious how this interacts with the rather complex rules of the conditional operator (11.4.11) for all possible edge cases.

Before merging a fix I'd like to have a more extensive set of test cases to make sure we fully understand what the standard specifies and to compare whatever fix we end up using with the behavior implemented by other tools.