CFG: In a DeclStmt, skip anything that's not a VarDecl.

Neither the compiler nor the analyzer are doing anything with non-VarDecl
decls in the CFG, and having them there creates extra nodes in the
analyzer's path diagnostics. Simplify the CFG (and the path edges) by
simply leaving them out. We can always add interesting decls back in when
they become relevant.

Note that this only affects decls declared in a DeclStmt, and then only
those that appear within a function body.

llvm-svn: 183157