This CL adds the Context object, variable ScopeStack and a Function map
into the type determiner.
The sample app is also updated to verify the module produced before
passing to the type determiner.
Bug: tint:5
Change-Id: Ib4af4e4305ee8a306f48e1bd328eaf3ad006fd9a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/18823
Reviewed-by: David Neto <dneto@google.com>