The CallDAG preprocesses the AST to construct a DAG of
functions that can be used for several analysis.
Use it to implement check for recursion and max call
depth. It will also be used to limit the usage of
[[flatten]] and [[unroll]].

BUG=angleproject:937
BUG=395048

Change-Id: I643e0ed605ad095e4fd7da4977d842be5e6a12e0
Reviewed-on: https://chromium-review.googlesource.com/229352Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>