[libc] move bcmp, bzero, bcopy, index, rindex, strcasecmp, strncasecmp to strings.h (#118899)

docgen relies on the convention that we have a file foo.cpp in
libc/src/\<header\>/. Because the above functions weren't in libc/src/strings/
but rather libc/src/string/, docgen could not find that we had implemented
these.

Rather than add special carve outs to docgen, let's fix up our sources for
these 7 functions to stick with the existing conventions the rest of the
codebase follows.

Link: #118860
Fixes: #118875

libc/fuzzing/string/CMakeLists.txt

@@ -38,5 +38,5 @@ add_libc_fuzzer(
   SRCS
     bcmp_fuzz.cpp
   DEPENDS
-    libc.src.string.bcmp
+    libc.src.strings.bcmp
 )

libc/fuzzing/string/bcmp_fuzz.cpp

@@ -9,7 +9,7 @@
 /// Fuzzing test for llvm-libc bcmp implementation.
 ///
 //===----------------------------------------------------------------------===//
-#include "src/string/bcmp.h"
+#include "src/strings/bcmp.h"
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>