QPermission: don't hide SFINAE constraints

Like done elsewhere (e.g. in QStringView), formulate the constaint in
if_x<T> = true from, and let qdoc see it. Then document the constraint
in prose, too.

As a drive-by, use C++17 variable templates instead of
std::bool_value, and document that data() returns a
default-constructed T on failure (which is indistinguishable from a
successful call that happens to return a default-constructed T, so we
should probably return an optional<T> here, but that's orthogonal to
the change at hand).

Pick-to: 6.5
Change-Id: I0584ce3f4febd619d3966afeb6244b11b167cd42
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

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