[libc++] Implement a libc++ private version of isascii (#122361)

The isascii() function is not standard, so we should avoid relying on
the platform providing it, especially since it's easy to implement in
libc++ portably.

GitOrigin-RevId: 4c6ca3efdae13a4dd75f9fe2cdfede5208e5d2c4
2 files changed