[libc++] Make sure we assume merged typeinfo names on Apple

The introduction of LIBCXX_HAS_MERGED_TYPEINFO_NAMES_DEFAULT changed
the default from =1 (assuming merged typeinfos) to =0 (not assuming
merged typeinfos) on all platforms where at least one other __config_site
macro is defined.

This commit explicitly enables the assumption of merged typeinfo names
on Apple platform to restore the previous behavior, at least until the
underlying issue has been fixed.

Author: ldionne

libcxx/cmake/caches/Apple.cmake

@@ -7,6 +7,7 @@ set(LIBCXX_ABI_VERSION "1" CACHE STRING "")
 set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_STATIC OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_SHARED ON CACHE BOOL "")
+set(LIBCXX_HAS_MERGED_TYPEINFO_NAMES_DEFAULT ON CACHE STRING "")
 set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
 set(LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS OFF CACHE BOOL "")
 set(LIBCXX_HIDE_FROM_ABI_PER_TU_BY_DEFAULT ON CACHE BOOL "")