Allow constructing values with their read-only bit on with a type-cast-expr <readonly>

[MaryamZi]

Description:
$title.

The spec says

Note that a type-cast-expr of can be used both to cause constructors withing the expression to construct values with the read-only bit on and to verify that the value resulting from the evaluation of expression has its read-only bit on.

Sub-task of #22515

[MaryamZi]

Moreover, the static type of <readonly> T should be T & readonly.

e.g.,

function foo(map<int> m) {
    map<int> & readonly n = <readonly> m; // valid
}

[MaryamZi]

The original issue will be fixed with #24484.

Moreover, the static type of <readonly> T should be T & readonly.

e.g.,

function foo(map<int> m) {
    map<int> & readonly n = <readonly> m; // valid
}

This will be tracked with #24559.