stdlib: use __swift_ssize_t instead of ssize_t (NFC)

`ssize_t` is not available on Windows.  We have an internal type definition
`__swift_ssize_t` which allows us to use the type.  Switch to that spelling.

Author: compnerd

stdlib/public/SwiftShims/LibcOverlayShims.h

@@ -114,7 +114,7 @@ int static inline _swift_stdlib_openat(int fd, const char *path, int oflag,
 }
 #endif
 
-static inline ssize_t
+static inline __swift_ssize_t
 _swift_stdlib_read(int fd, void *buf, size_t nbyte) {
 #if defined(_WIN32)
   return _read(fd, buf, nbyte);
@@ -123,7 +123,7 @@ _swift_stdlib_read(int fd, void *buf, size_t nbyte) {
 #endif
 }
 
-static inline ssize_t
+static inline __swift_ssize_t
 _swift_stdlib_write(int fd, const void *buf, size_t nbyte) {
 #if defined(_WIN32)
   return _write(fd, buf, nbyte);