Fix "use std::launder" predicate for Android NDKs

Summary:
The current predicate yields false positive for NDK r17. NDK r18 has launder without defining `__cpp_lib_launder`. NDK r19 finally gets it right by defining `__cpp_lib_launder`.

r17: launder? no,  `__cpp_lib_launder` undefined, `_LIBCPP_VERSION == 6000`
r18: launder? yes, `__cpp_lib_launder` undefined, `_LIBCPP_VERSION == 7000`
r19: launder? yes, `__cpp_lib_launder == 201606L`, `_LIBCPP_VERSION == 8000`

Reviewed By: yfeldblum

Differential Revision: D14098189

fbshipit-source-id: 315dba44c6474407a8f4c1d25c81fff057d5f7db

folly/lang/Launder.h

@@ -28,7 +28,8 @@
 
 // Note: libc++ 6+ adds std::launder but does not define __cpp_lib_launder
 #if __cpp_lib_launder >= 201606 || (_MSC_VER && _HAS_LAUNDER) || \
-    (_LIBCPP_VERSION >= 6000 && __cplusplus >= 201703L)
+    ((_LIBCPP_VERSION >= (__ANDROID__ ? 7000 : 6000)) &&         \
+     __cplusplus >= 201703L)
 
 namespace folly {