[libunwind] Provide placement new definition

While Clang automatically generates the code for placement new,
g++ doesn't do that so we need to provide our own definition.

Differential Revision: https://reviews.llvm.org/D57455

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@352966 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed