[libc] Update size_t and ssize_t definitions to use __SIZE_TYPE__ and __PTRDIFF_TYPE__ respectively. (#143921)

The current definition of `ssize_t` does not have the same bit width as
`size_t` on 32-bit platforms.

libc/include/llvm-libc-types/size_t.h

@@ -9,11 +9,6 @@
 #ifndef LLVM_LIBC_TYPES_SIZE_T_H
 #define LLVM_LIBC_TYPES_SIZE_T_H
 
-// Since __need_size_t is defined, we get the definition of size_t from the
-// standalone C header stddef.h. Also, because __need_size_t is defined,
-// including stddef.h will pull only the type size_t and nothing else.
-#define __need_size_t
-#include <stddef.h>
-#undef __need_size_t
+typedef __SIZE_TYPE__ size_t;
 
 #endif // LLVM_LIBC_TYPES_SIZE_T_H

libc/include/llvm-libc-types/ssize_t.h

@@ -9,6 +9,6 @@
 #ifndef LLVM_LIBC_TYPES_SSIZE_T_H
 #define LLVM_LIBC_TYPES_SSIZE_T_H
 
-typedef __INT64_TYPE__ ssize_t;
+typedef __PTRDIFF_TYPE__ ssize_t;
 
 #endif // LLVM_LIBC_TYPES_SSIZE_T_H