Remove not needed call to __is_long()

[nilayvaish]

Summary

The string is known to be long since grow_by unconditionally calls
set_long_cap().

[• Quuxplusone]

Seems right to me, as long as CI is green. (But wait for someone to "accept as libc++" before you land this.)
If you don't have commit rights, then someone else will have to land it; in that case please provide your full name and email address, here.

[ldionne]

Will edit as-of committing.

libcxx/include/string

2661

libcxx/include/string

basic_string<_CharT, _Traits, _Allocator>::push_back(value_type __c)
     else
     {
         __cap = __get_long_cap() - 1;
         __sz = __get_long_size();
     }
     if (__sz == __cap)
     {
         __grow_by(__cap, 1, __sz, __sz, 0);
-        __is_short = !__is_long();
+        __is_short = false;
     }
     pointer __p;
     if (__is_short)
     {
         __p = __get_short_pointer() + __sz;
         __set_short_size(__sz+1);
     }
     else