[analyzer] Add scope information to CFG

This patch adds two new CFG elements CFGScopeBegin and CFGScopeEnd that indicate
when a local scope begins and ends respectively. We use first VarDecl declared
in a scope to uniquely identify it and add CFGScopeBegin and CFGScopeEnd elements
into corresponding basic blocks.

Differential Revision: https://reviews.llvm.org/D16403

llvm-svn: 327258