Enhance LiveVariables to understand that blocks can extend the liveness of a variable...

Enhance LiveVariables to understand that blocks can extend the liveness of a variable by "capturing" them in a BlockExpr.

This required two changes:

1) Added 'getReferencedgetReferencedBlockVars()' to AnalysisContext so
that clients can iterate over the "captured" variables in a block.

2) Modified LiveVariables to take an AnalysisContext& in its
constructor and to call getReferencedgetReferencedBlockVars() when it
processes a BlockExpr*.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@89924 91177308-0d34-0410-b5e6-96231b3b80d8