These should exist in all toolchains LLVM supports nowadays.
Enables making DataTypes.h a regular header instead of a .h.cmake file and
allows deleting a bunch of cmake goop (which should also speed up cmake
configure time a bit).
All the code this removes is 9+ years old.
https://reviews.llvm.org/D45155
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328970 91177308-0d34-0410-b5e6-96231b3b80d8
