[perl #108780] Make ‘no overloading’ work with qr//

[perl #108780] Make ‘no overloading’ work with qr//

Traditionally, overload::StrVal(qr//) has returned
Regexp=SCALAR(0xc0ffee), and later Regexp=REGEXP(0xc0c0a) when regexps
were made into first-class SVs.

When the overloading pragma was added in 5.10.1, qr// things were not
accounted for, so they would still stringify as (?-xism:) even with
‘no overloading’ (or as (?^:) under 5.14).

This commit makes the overloading pragma work with qr// things, so
that they stringify the same way as overload::StrVal; i.e., as
Regexp=REGEXP(0xbe600d).