<format>: Incorrect decimal point placement when using alternate form without a type

@fsb4000 reported a bug in libc++'s format implementation. While investigating that bug I noticed that for a small part I'm in good company; MSVC STL has the same bug:
```
std::format("{:#.{}}\n", 1200.0 0);  // 1e+03. instead of 1.e+03
std::format("{:#.{}g}\n", 1200.0 0); // 1.e+03 as expected (types a, e, and f are good too.)
```
More examples at https://godbolt.org/z/fTsKaf6vz