[analyzer] Add scope information to CFG

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@327258 91177308-0d34-0410-b5e6-96231b3b80d8