[patch] [libcxx] _LIBCPP_WEAK

[Nico Rieck]

_LIBCPP_NEW_DELETE_VIS expands to _LIBCPP_FUNC_VIS_ONLY when using 
libc++ (which in turn expands to nothing on Windows). When building 
libc++ as shared library it expands to _LIBCPP_FUNC_VIS (which in turn 
expands to dllexport). The new/delete operators are special cases that 
must not be decorated with dllimport so that they can be replaced with a 
local non-imported definition.

-Nico