libcxx: Use vcruntime declarations for typeinfo on Windows.

We need to use the vcruntime declarations on Windows to avoid an
ODR violation involving rtti.obj, which provides the definition of
the runtime function implementing dynamic_cast and depends on the
vcruntime implementations of bad_cast and bad_typeid.

Differential Revision: https://reviews.llvm.org/D42220

llvm-svn: 323491