Nested `this` register is not populated if only access is via `getMember`.

[FeepingCreature]

This segfaults in LDC 1.42.0:

void main() { Struct().get; }
struct Struct {
    int field;
    int get() {
        int get2() {
            // cast(void) this;
            return __traits(getMember, this, "field");
        }
        return get2;
    }
}

See: https://godbolt.org/z/1joxsYPWq

If this is explicitly accessed, it works.

edit: Reported to also happen in LDC 1.41!

edit: And on master, alright.

[niy-ati]

Opened #5205.

Root cause: traits(getMember) with wantsym resolves to the field VarDeclaration, so nested functions never recorded a nested reference to the enclosing member this. LDC then built the closure without vthis in the frame (nestedIndex -1 / invalid storage), which matches the segfault when the returned nested delegate runs.

The fix calls checkNestedReference on vthis after getMember when the lookup object is this. Added tests/dmd/runnable/issue5203.d with the delegate+getMember pattern from this report.