Move Simple.h and Alias.h to include/Core.

Because the files in Core actually depend on these files.

Differential Revision: http://reviews.llvm.org/D4000

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@210710 91177308-0d34-0410-b5e6-96231b3b80d8