The CallDAG preprocesses the AST to construct a DAG of
functions that can be used for several analyses.
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: I8578703f2d49513f315aecccbcff34914562e4ff
Reviewed-on: https://chromium-review.googlesource.com/263774Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>