[PATCH] [libc++] Support newlib as libc++'s C library [locale part]

[Joerg Sonnenberger]

Why don't we fix the real problem first and stop bothering with the symptoms. The isascii checks break the masks for any char with high bit set. We should IMO have a templated function map2index or so. Then we get:
```
size_t map2index<char>(char_type a) { return (unsigned char)a; }
size_t map2index<wchar_t>(char_type a) { return a; }
```

http://reviews.llvm.org/D5385