Revert "Change the ABI version and ABI namespace to be `_LIBCPP_VERSION`"

There are some suspicious bot failures that I want to ensure aren't
caused by this patch.

I'll recommit tomorrow.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@364363 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed