[libc] implement iswctype

[michaelrj-google]

Needed for libc++ wchar support. Defined on page 467 of the C standard: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf#page=467

[llvmbot]

@llvm/issue-subscribers-libc

Author: Michael Jones (michaelrj-google)

Details

Needed for libc++ wchar support. Defined on page 467 of the C standard: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf#page=467

[xiongzile]

I’d like to work on this.