[perl #108780] Make /foo$qr/ work under ‘no overloading’

[perl #108780] Make /foo$qr/ work under ‘no overloading’

This commit redoes 37c07a4b2, which had to be reverted to allow the
re-eval work to be rebased onto blead.

It changes the code in re_op_compile (formerly in pp_regcomp) to use
the underlying REGEXP instead of the reference to it, when concatenat-
ing pieces to mark a larger regular expression.  This makes /foo$qr/
work even under ‘no overloading’.  It stopped working with commit
a75c6ed6b.