Fix PR#31454 - 'basic_string<T>::push_back() crashes if sizeof(T)>sizeof(long long)'. We were mishandling the small-string optimization calculations for very large 'characters'. This may be an ABI change (change the size of) strings of very large 'characters', but since they never worked, I'm not too concerned.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324531 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.modifiers/string_append/push_back.pass.cpp

@@ -48,7 +48,7 @@ int main()
     test(S("12345678901234567890"), 'a', S("12345678901234567890a"));
     }
 #endif
-#if 0
+
     {
 // https://bugs.llvm.org/show_bug.cgi?id=31454
     std::basic_string<veryLarge> s;
@@ -57,5 +57,4 @@ int main()
     s.push_back(vl);
     s.push_back(vl);
     }
-#endif
 }