Refactor 'PostStmt' and 'PreStmt' to subclass a common parent 'StmtPoint'.

Educate GRExprEngine::VisitGraph() about 'PreStmt'.

Mark the constructor of 'PostStmt' to be explicit, preventing implicit
conversions and the selection of the wrong 'generateNode' method in
GRStmtNodeBuilder.

Constify a bunch of arguments, which falls out of the changes to ProgramPoint.

llvm-svn: 76809