Assume existence of inttypes.h and stdint.h in DataTypes.h.

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

llvm-svn: 328970