eval"" should reset %^H in more cases

eval"" should reset %^H in more cases

This is scary:

    #use sort 'stable';
    require re; re->import('/x');
    eval '
      print "a b" =~ /a b/ ? "ok\n" : "nokay\n";
      use re "/m";
      print "a b" =~ /a b/ ? "ok\n" : "nokay\n";
   ';

It prints:

    ok
    nokay

The re->import statement is supposed to apply to the caller that
is currently being compiled, but it makes ‘use re "/m"’ enable
/x as well.

Uncomment the ‘use sort’ line, and you get:

    ok
    ok

which is even scarier.

eval"" is supposed to compile its argument with the hints under which
the eval itself was compiled.

Whenever %^H is modified, a flag (HINT_LOCALIZE_HH; LHH hereinafter)
is set in $^H.

When eval is called, it checks the LHH flag in the hints from the time
it was compiled, to determine whether to reset %^H.  If LHH is set,
it creates a new %^H based on the hints under which it was compiled.
Otherwise, it just leaves %^H alone.

The problem is that %^H and LHH may be set some time later
(re->import), so when the eval runs there is junk in %^H that
does not apply to the contents of the eval.

There are two layers at which the hints hash is stored.  There is the
Perl-level hash, %^H, and then there is a faster cop-hints-hash struc-
ture underneath.  It’s the latter that is actually used during compi-
lation.  %^H is just a Perl front-end to it.

When eval does not reset %^H and %^H has junk in it, the two get
out of sync, because eval always sets the cop-hints-hash correctly.
Hence the first print in the first example above compiles without
‘use re "/x"’.  The ‘use re’ statement after it modifies the %^H-with-
junk-in-it, which then gets synchronised with the cop-hints-hash,
turning on /x for the next print statement.

Adding ‘use sort’ to the top of the program makes the problem go
away, because, since sort.pm uses %^H, LHH is set when eval() itself
is compiled.

This commit fixes this by having pp_entereval check not only the LHH
flag from the hints under which it was compiled, but also the hints of
the currently compiling code ($^H / PL_hints).