[PPCG] [2/3] Make polly specific PPCG Changes.

- This commit *WILL NOT COMPILE*. `PPCGCodeGeneration` requires changes
  since some of PPCG's internal data structures have been modified.

- Has polly-speific changes to PPCG. Polly exports certain functionality that
  is private to PPCG. It also creates stubs for large parts of the pet API as
  well as other functions in `ppcg/external.c` to keep the linker happy.

- This commit includes changes to CMakeLists.txt.

Differential Revision: https://reviews.llvm.org/D35676

llvm-svn: 308624