libstdc++: Improve list assumption after constructor [PR118865]

libstdc++: Improve list assumption after constructor [PR118865]

The code example here does:
```
if (begin == end) __builtin_unreachable();
std::list nl(begin, end);

for (auto it = nl.begin(); it != nl.end(); it++)
{
...
}
/* Remove the first element of the list. */
nl.erase(nl.begin());
```

And we get a warning because because we jump threaded the case were we
think the list was empty from the for loop BUT we populated it without
an empty array. So can help the compiler here by adding that after initializing
the list with non empty array, that the list will not be empty either.

This is able to remove the -Wfree-nonheap-object warning in the first reduced
testcase (with the fix for `begin == end` case added) in the PR 118865; the second
reduced testcase has been filed off as PR 118867.

Bootstrapped and tested on x86_64-linux-gnu.

libstdc++-v3/ChangeLog:

	PR libstdc++/118865
	* include/bits/stl_list.h (_M_initialize_dispatch): Add an
	unreachable if the iterator was not empty that the list will
	now be not empty.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>