Speculative buildfix for Mac

Our builds were failing with

FAILED: lib/Support/CMakeFiles/LLVMSupport.dir/ARMBuildAttrs.cpp.o
[..]
In file included from /b/c/b/ToTMac/src/third_party/llvm/lib/Support/ARMBuildAttrs.cpp:9:
In file included from /b/c/b/ToTMac/src/third_party/llvm/include/llvm/ADT/StringRef.h:12:
In file included from /b/c/b/ToTMac/src/third_party/llvm/include/llvm/ADT/STLExtras.h:19:
/b/c/b/ToTMac/src/third_party/llvm/include/llvm/ADT/Optional.h:88:25: error: no member named 'addressof' in namespace 'std'
    ::new ((void *)std::addressof(value)) T(std::forward<Args>(args)...);
                   ~~~~~^

Try to fix by including <memory>

llvm-svn: 354472