Rework CFG edges to encode potentially unreachable edges, instead of just making...

Rework CFG edges to encode potentially unreachable edges, instead of just making them NULL.

This is to support some analyses, like -Wunreachable-code, that
will need to recover the original unprunned CFG edges in order
to suppress issues that aren't really bugs in practice.

There are two important changes here:

- AdjacentBlock replaces CFGBlock* for CFG successors/predecessors.
  This has the size of 2 pointers, instead of 1.  This is unlikely
  to have a significant memory impact on Sema since a single
  CFG usually exists at one time, but could impact the memory
  usage of the static analyzer.  This could possibly be optimized
  down to a single pointer with some cleverness.

- Predecessors can now contain null predecessors, which means
  some analyses doing a reverse traversal will need to take into
  account.  This already exists for successors, which contain
  successor slots for specific branch kinds (e.g., 'if') that
  expect a fixed number of successors, even if a branch is
  not reachable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202325 91177308-0d34-0410-b5e6-96231b3b80d8