[libc] split strings.h from string.h

[nickdesaulniers]

bcmp, bzero, bcopy, index, rindex, strcasecmp, and strncasecmp are technically supposed to be from strings.h, not string.h.

If we move these into a src/strings/ dir, then docgen can find that we have them implemented.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

bcmp, bzero, bcopy, index, rindex, strcasecmp, and strncasecmp are technically supposed to be from strings.h, not string.h.

If we move these into a src/strings/ dir, then docgen can find that we have them implemented.