[CodeGen] Clean up access to EmittedDeferredDecls, NFCI.

GlobalDecls should only be added to EmittedDeferredDecls if they
need reemission. This is checked in addEmittedDeferredDecl, which
is called via addDeferredDeclToEmit. Extend these checks to also
handle VarDecls (for lambdas, as tested in Interpreter/lambda.cpp)
and remove the direct access of EmittedDeferredDecls in EmitGlobal
that may actually end up duplicating FunctionDecls.

Differential Revision: https://reviews.llvm.org/D156897