[Sema] Correctly re-contextualize `if`/`switch` exprs in lazy vars
With `if`/`switch` expressions, we may now have
local bindings within lazy initializers, and
therefore need to ensure we correctly
re-contextualize them. Adjust the walker to set
the DeclContext for all decls it encounters,
and make sure it handles some cases that the
ASTWalker does not currently visit.
rdar://119158202