[CFG] Fix an assertion failure with static initializers

The CFGBlock::getLastCondition was not prepared for static initializer
branches.

This patch also revamps CFG unit tests. Earlier the lifetime of the AST
was smaller than the CFG. So all the AST pointers within the CFG blocks
were dangling. This was OK, since none of the tests dereferenced those
pointers. This was, however, a timed bomb. There were patches in the
past that were reverted partially due to this problem.

Differential revision: https://reviews.llvm.org/D71791