bpo-35214: Annotate posix calls for clang MSan. (#11389)

It doesn't know the details of a few less common libc functions.

Misc/NEWS.d/next/Core and Builtins/2018-12-30-15-36-23.bpo-35214.GWDQcv.rst

 clang Memory Sanitizer build instrumentation was added to work around false
-positives from socket, time, test_io, and test_faulthandler.
+positives from posix, socket, time, test_io, and test_faulthandler.

Modules/posixmodule.c

@@ -367,6 +367,10 @@ static int win32_can_symlink = 0;
 #define HAVE_STRUCT_STAT_ST_FSTYPE 1
 #endif
 
+#ifdef _Py_MEMORY_SANITIZER
+# include <sanitizer/msan_interface.h>
+#endif
+
 #ifdef HAVE_FORK
 static void
 run_at_forkers(PyObject *lst, int reverse)
@@ -5493,6 +5497,9 @@ os_posix_spawn_impl(PyObject *module, path_t *path, PyObject *argv,
         PyErr_SetFromErrnoWithFilenameObject(PyExc_OSError, path->object);
         goto exit;
     }
+#ifdef _Py_MEMORY_SANITIZER
+    __msan_unpoison(&pid, sizeof(pid));
+#endif
     result = PyLong_FromPid(pid);
 
 exit:
@@ -6098,6 +6105,9 @@ os_sched_rr_get_interval_impl(PyObject *module, pid_t pid)
         posix_error();
         return -1.0;
     }
+#ifdef _Py_MEMORY_SANITIZER
+    __msan_unpoison(&interval, sizeof(interval));
+#endif
     return (double)interval.tv_sec + 1e-9*interval.tv_nsec;
 }
 #endif /* HAVE_SCHED_RR_GET_INTERVAL */
@@ -6568,6 +6578,12 @@ posix_getgrouplist(PyObject *self, PyObject *args)
         return posix_error();
     }
 
+#ifdef _Py_MEMORY_SANITIZER
+    /* Clang memory sanitizer libc intercepts don't know getgrouplist. */
+    __msan_unpoison(&ngroups, sizeof(ngroups));
+    __msan_unpoison(groups, ngroups*sizeof(*groups));
+#endif
+
     list = PyList_New(ngroups);
     if (list == NULL) {
         PyMem_Del(groups);