[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

llvm-svn: 352966
GitOrigin-RevId: 5745e908004c5242c62e24bb94d7d13f375aae6a
3 files changed