[MSan] Add COMMON_INTERCEPTOR_NOTHING_IS_INITIALIZED

[iii]

Summary

MSan not implementing COMMON_INTERCEPTOR_NOTHING_IS_INITIALIZED looks
like an omission - this macro makes it possible for those intercepted
functions, which libc needs very early, to work before interceptors are
initialized (i.e. before REAL() is usable).

While currently there are no observable practical problems in this
area, changes in libc or in MSan runtime may provoke them. Therefore,
change MSan to work like ASan and TSan already do - use internal
functions in certain interceptors when initialization is not complete.

[eugenis]

LGTM

compiler-rt/lib/msan/msan_interceptors.cpp

 #define COMMON_INTERCEPTOR_ON_EXIT(ctx) OnExit()
 #define COMMON_INTERCEPTOR_LIBRARY_LOADED(filename, handle)                    \
   do {                                                                         \
     link_map *map = GET_LINK_MAP_BY_DLOPEN_HANDLE((handle));                   \
     if (filename && map)                                                       \
       ForEachMappedRegion(map, __msan_unpoison);                               \
   } while (false)
 
+#define COMMON_INTERCEPTOR_NOTHING_IS_INITIALIZED (!msan_inited)
+
 #define COMMON_INTERCEPTOR_GET_TLS_RANGE(begin, end)                           \
   if (MsanThread *t = GetCurrentThread()) {                                    \
     *begin = t->tls_begin();                                                   \
     *end = t->tls_end();                                                       \
   } else {                                                                     \
     *begin = *end = 0;                                                         \
   }