Fix some Wundef warnings in Compiler.h

Check for definedness of the __cpp_sized_deallocation and
__cpp_aligned_new feature test macros.  These will not be defined
when the feature is not available, and that prevents any code that
includes this header from compiling with -Wundef -Werror.

Differential Revision: https://reviews.llvm.org/D51171

llvm-svn: 341364