[PPCG] [1/3] Bump up PPCG version to 0.07.

- This commit *WILL NOT COMPILE*, as it checks in vanilla PPCG 0.07
- We choose to introduce this commit into the history to cleanly display
  the Polly-specific changes made to PPCG.

Differential Revision: https://reviews.llvm.org/D35675

llvm-svn: 308623