Transform clang::DominatorTree to be able to also calculate post dominators.
- Tidy up the documentation
- Make it clang::DominatorTree template class (similarly to how llvm::DominatorTreeBase works), rename it to clang::CFGDominatorTreeImpl
- Clang's dominator tree is now called clang::CFGDomTree
- Clang's brand new post dominator tree is called clang::CFGPostDomTree
- Add a lot of asserts to the dump() function
- Create a new checker to test the functionality