Fixes based on the code review

(1) Renamed to `_LIBCPP_IF_ASAN`.
(2) Moved to `__config`.
(3) Added `{ ... }` after `_LIBCPP_IF_ASAN()`.

Missing: `#ifdef` around ASan related variables.

Author: Advenam Tacet

libcxx/include/__config

@@ -1490,6 +1490,16 @@ __sanitizer_verify_double_ended_contiguous_container(const void*, const void*, c
 #    define _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 #  endif
 
+// ASan related macros
+// _LIBCPP_IF_ASAN is used to removed ASan related code, whenever compiled without ASan.
+#ifndef _LIBCPP_IF_ASAN
+#  ifndef _LIBCPP_HAS_NO_ASAN
+#    define _LIBCPP_IF_ASAN() if _LIBCPP_CONSTEXPR (true)
+#  else
+#    define _LIBCPP_IF_ASAN() if _LIBCPP_CONSTEXPR (false)
+#  endif
+#endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP___CONFIG