Fix std::pointer_safety type in ABI v2

In the C++ standard `std::pointer_safety` is defined
as a C++11 strongly typed enum. However libc++ currently defines
it as a class type which simulates a C++11 enumeration. This
can be detected in valid C++ code.

This patch introduces an the _LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE ABI option.
When defined `std::pointer_safety` is implemented as an enum type.
Unfortunatly this also means it can no longer be provided as an extension
in C++03.

Additionally this patch moves the definition for `get_pointer_safety()`
out of the dylib, and into the headers. New usages of `get_pointer_safety()`
will now use the inline version instead of the dylib version. However in
order to keep the dylib ABI compatible the old definition is explicitly
compiled into it.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291046 91177308-0d34-0410-b5e6-96231b3b80d8

src/memory.cpp

@@ -220,11 +220,12 @@ undeclare_no_pointers(char*, size_t)
 {
 }
 
-pointer_safety
-get_pointer_safety() _NOEXCEPT
+#if !defined(_LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE)
+pointer_safety get_pointer_safety() _NOEXCEPT
 {
     return pointer_safety::relaxed;
 }
+#endif
 
 void*
 __undeclare_reachable(void* p)