[analyzer][Dominator] Add post dominators to CFG + a new debug checker

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

Differential Revision: https://reviews.llvm.org/D62551

llvm-svn: 365028