- Thread Type::FunctionType through stubs and the TF pipeline.
- Augment Typer to decorate parameter nodes with types from
  a Type::FunctionType associated with interface descriptors.
- Factor interface descriptors into platform-specific and
  platform-independent components so that all descriptors share
  a common Type::FunctionType for all platforms.

Review URL: https://codereview.chromium.org/1197703002

Cr-Commit-Position: refs/heads/master@{#29248}