[libc] Use anonymous namespace for file-local symbols (#157202)

A namespace like LIBC_NAMESPACE::internal should only ever be
defined if it's providing global symbols declared in headers.
These StringUtil implementations were defining global namespaced
symbols for their file-local helper code, which they should not.

GitOrigin-RevId: 4d0b81616975c4ea8984170122a04553cb14337a