bpo-35214: Add _Py_ prefix to MEMORY_SANITIZER def. (GH-10503)


Rename our new MEMORY_SANITIZER define to _Py_MEMORY_SANITIZER.
Project based C Preprocessor namespacing at its finest. :P
(cherry picked from commit 3015fb8ce4d25603434b9b44bb7effb98a481532)

Co-authored-by: Gregory P. Smith <greg@krypto.org>
diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index a49194e..ed4e12b 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -3,7 +3,7 @@
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
 
-#if defined(HAVE_GETC_UNLOCKED) && !defined(MEMORY_SANITIZER)
+#if defined(HAVE_GETC_UNLOCKED) && !defined(_Py_MEMORY_SANITIZER)
 /* clang MemorySanitizer doesn't yet understand getc_unlocked. */
 #define GETC(f) getc_unlocked(f)
 #define FLOCKFILE(f) flockfile(f)