[libc++] Introduce `_LIBCPP_OVERRIDABLE_FUNC_VIS`

This is a generalization of `_LIBCPP_NEW_DELETE_VIS`; the new macro name
captures the semantics better, and also allows us to get rid of the
`_WIN32` check in `include/new`. No functional change.

Differential Revision: https://reviews.llvm.org/D26702

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@287164 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed