Add CFG logic to create a conditional branch for modeling static initializers.

Add CFG logic to create a conditional branch for modeling static initializers.

This is an optional variant of the CFG.  This allows analyses to model whether
or not a static initializer has run, e.g.:

  static Foo x = bar();

For basic dataflow analysis in Sema we will just assume that the initializer
always runs.  For the static analyzer we can use this branch to accurately
track whether or not initializers are on.

This patch just adds the (opt-in) functionality to the CFG.  The
static analyzer still needs to be modified to adopt this feature.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178263 91177308-0d34-0410-b5e6-96231b3b80d8