Rename the static version of libc++ on Windows.

Previously both the static version of libc++ and the
import library for the DLL had the same name, 'c++.lib'.

This patch renames the static library on Windows to be `libc++.lib`
so it no longer conflicts. This naming convention is consistent with
other windows libraries.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300817 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed