Cleanup header dependencies of llvm/Support/Compiler.h

<new> and <cstdef> were introduced in aa60b3fd but the dependency
is now dead.

As a consequence you may need to include <new> where you use it while it
was auto-included as an implicit dependency before.

The impact on the codebase is small, as <new> is a very small header
(<100 SLOC) but it gets included everywhere, so that somehow counts (?)