Warn instead of DIE when a repetition would exhaust RAM

For discussions on #23561.

perl -e 'use warnings; my $x = ($_) ? "A" x (2**62) : "Z"'

gives this on blead for me:
```
Out of memory!
panic: fold_constants JMPENV_PUSH returned 2 at -e line 1.
```

on the previous commit, it would die:
```
Unrealistically large string repetition value"
```

With this commit, it just warns:
```
Unrealistically large string repetition value at -e line 1.
```

but will blow up if the repetition OP does get executed:
```
Out of memory in perl:util:safesysrealloc
```

Author: richardleach

op.c

@@ -5025,20 +5025,19 @@ S_fold_constants(pTHX_ OP *const o)
             if (SvIOKp(constsv)) {
                 if (SvIOK_UV(constsv)) {
                     if (SvUVX(constsv) > SIZE_MAX >> 2)
-                        goto repetition_die;
+                        ck_warner(packWARN(WARN_MISC), "Unrealistically large string repetition value");
                     if (SvUVX(constsv) > arbitrary)
                         goto nope;
                 } else {
                     if (SvIVX(constsv) > (IV)(SIZE_MAX >> 2))
-                        goto repetition_die;
+                        ck_warner(packWARN(WARN_MISC), "Unrealistically large string repetition value");
                     if (SvIVX(constsv) > (IV)arbitrary)
                         goto nope;
                 }
             } else {
-                NV rhs = SvNV_nomg(constsv);
-                if (rhs > (NV)(SIZE_MAX >> 2)) {
-                  repetition_die:
-                    DIE(aTHX_ "Unrealistically large string repetition value");
+                NV rhs = 0.0; rhs = SvNV_nomg(constsv);
+                if (rhs >= (NV)((SIZE_MAX >> 2) +1) ) {
+                    ck_warner(packWARN(WARN_MISC), "Unrealistically large string repetition value");
                 }
                 if (rhs > (NV)arbitrary)
                     goto nope;

pod/perldiag.pod

@@ -7387,7 +7387,7 @@ subroutine.
 
 =item Unrealistically large string repetition value
 
-The value of the right operand in the string repetition operator is
+(W misc) The value of the right operand in the string repetition operator is
 likely close to or will exceed the maximum memory allocation that
 your system can provide.
 

t/op/repeat.t

@@ -195,10 +195,10 @@ is($@, "", "RT #130247");
 
 # [GH #13324] Perl croaks if a string repetition seems unsupportable
 fresh_perl_like(
- 'my $x = "A" x (2**99)',
+ 'use warnings; my $x = "A" x (2**99)',
   qr/Unrealistically large string repetition/,
-  {  },
- 'Croak on unrealistically large string repetition',
+  {stderr => 1},
+ 'Warn on unrealistically large string repetition',
 );
 
 # yes, the newlines matter