[libc++] Make __shared_weak_count vtable consistent across all build configurations

This patch ensures that __shared_weak_count provides a consistent vtable
regardless of if RTTI is enabled or if we are targeting a static or shared
libc++ build.

This patch is technically ABI breaking, but only for a very specific
configuration that no vendor should be shipping.

Note that _LIBCPP_BUILD_STATIC is not normally defined when building
libc++.a, but instead it must be manually provided by the user or the
__config_site.

Differential Revision: https://reviews.llvm.org/D32838

libcxx/include/memory

@@ -3271,13 +3271,7 @@ public:
     long use_count() const _NOEXCEPT {return __shared_count::use_count();}
     __shared_weak_count* lock() _NOEXCEPT;
 
-    // Define the function out only if we build static libc++ without RTTI.
-    // Otherwise we may break clients who need to compile their projects with
-    // -fno-rtti and yet link against a libc++.dylib compiled
-    // without -fno-rtti.
-#if !defined(_LIBCPP_NO_RTTI) || !defined(_LIBCPP_BUILD_STATIC)
     virtual const void* __get_deleter(const type_info&) const _NOEXCEPT;
-#endif
 private:
     virtual void __on_zero_shared_weak() _NOEXCEPT = 0;
 };

libcxx/src/memory.cpp

@@ -124,16 +124,12 @@ __shared_weak_count::lock() _NOEXCEPT
     return nullptr;
 }
 
-#if !defined(_LIBCPP_NO_RTTI) || !defined(_LIBCPP_BUILD_STATIC)
-
 const void*
 __shared_weak_count::__get_deleter(const type_info&) const _NOEXCEPT
 {
     return nullptr;
 }
 
-#endif  // _LIBCPP_NO_RTTI
-
 #if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 
 _LIBCPP_SAFE_STATIC static const std::size_t __sp_mut_count = 16;

libcxxabi/CMakeLists.txt

@@ -338,10 +338,6 @@ else()
     list(APPEND LIBCXXABI_COMPILE_FLAGS -DNDEBUG)
   endif()
 endif()
-# Static library
-if (NOT LIBCXXABI_ENABLE_SHARED)
-  list(APPEND LIBCXXABI_COMPILE_FLAGS -D_LIBCPP_BUILD_STATIC)
-endif()
 
 # Threading
 if (NOT LIBCXXABI_ENABLE_THREADS)