- Name lookup in module scopes has namespace semantics now: All
  overloads from all parent modules are combined before overload
  resolution.
- Allow overloads of different callables: runtime-functions,
  macros, builtins, and generics.
- The duplication between the DeclarationVisitor and the
  ImplementationVisitor is removed: The DeclarationVisitor creates
  declarables for everything except for implicit generic specializations.
  The ImplementationVisitor iterates over declarables.
  The DeclarationVisitor only looks at the header of declarations, not
  at the body.
- Modules become Declarable's, which will enable them to be nested.
- Modules replace the existing Scope chain mechanism, which will make it
  easier to inline macros.
- The DeclarationVisitor and Declarations become stateless. All state is
  moved to contextual variables and the GlobalContext.
- Implicit specializations are created directly from the
  ImplementationVisitor. This will enable template parameter inference.
- As a consequence, the list of all builtins is only available after the
  ImplementationVisitor has run. Thus GenerateBuiltinDefinitions has to
  move to the ImplementationVisitor. Also, this makes it necessary to
  resolve the link from function pointer types to example builtins only
  at this point.


Bug: v8:7793
Change-Id: I61cef2fd3e954ab148c252974344a6e38ee2d01d
Reviewed-on: https://chromium-review.googlesource.com/c/1304294
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57231}