[libunwind] Drop the dependency on <algorithm>, add placement new inline

We haven't eliminated C++ library dependency altogether in D57251,
UnwindCursor.hpp had an unused dependency on <algorithm> which was
pulling in other C++ headers. Removing that dependency also revealed
(correctly) that we need our own global placement new declaration. Now
libunwind should be independent of the C++ library.

Differential Revision: https://reviews.llvm.org/D57262

llvm-svn: 352384