[clang][IFS] Prevent Clang-IFS from Leaking symbols from inside a block.

Built libdispatch with clang interface stubs. Ran into some block
related issues. Basically VarDecl symbols can leak out because I wasn't
checking the case where a VarDecl is contained inside a BlockDecl
(versus a method or function).

This patch checks that a VarDecl is not a child decl of a BlockDecl.
This patch also does something very similar for c++ lambdas as well.

Differential Revision: https://reviews.llvm.org/D71301