[libc++][CMake] Always provide new/delete in libc++ unless specified otherwise

[ldionne]

Summary

Let's not try to be clever and detect it based on the libc++abi setting.
The only build that puts new/delete in libc++abi is Apple's and we set
this CMake option explicitly in both libc++ and libc++abi. Complicated
dependent options hurt, let's avoid them when possible.

[phosek]

LGTM

libcxx/CMakeLists.txt

if (LLVM_HAVE_LINK_VERSION_SCRIPT AND NOT LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY
       AND LIBCXX_ENABLE_SHARED)
     set(ENABLE_LINKER_SCRIPT_DEFAULT_VALUE ON)
 endif()
 
 option(LIBCXX_ENABLE_ABI_LINKER_SCRIPT
       "Use and install a linker script for the given ABI library"
       ${ENABLE_LINKER_SCRIPT_DEFAULT_VALUE})
 
-set(ENABLE_NEW_DELETE_DEFAULT ON)
-if (LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS)
-# FIXME: This option should default to off. Unfortunatly GCC 4.9 fails to link
-# programs due to undefined references to new/delete in libc++abi so to work
-# around this libc++abi currently defaults LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS
-# to ON. Once the GCC bug has been worked around this option should be changed
-# back to OFF.
-  set(ENABLE_NEW_DELETE_DEFAULT ON)
-endif()
-
 option(LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS
     "Build libc++ with definitions for operator new/delete. This option can
      be used to disable the definitions when libc++abi is expected to provide
-    them" ${ENABLE_NEW_DELETE_DEFAULT})
+     them" ON)
 
 # Build libc++abi with libunwind. We need this option to determine whether to
 # link with libunwind or libgcc_s while running the test cases.
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
 
 # Target options --------------------------------------------------------------
 option(LIBCXX_BUILD_32_BITS "Build 32 bit libc++." ${LLVM_BUILD_32_BITS})