Annotate call nodes in the TF graph with source code information in the form
of byte offset relative to the wasm function start. The backend finally outputs those positions as RelocInfo.

R=bmeurer@chromium.org, mstarzinger@chromium.org, titzer@chromium.org

Review URL: https://codereview.chromium.org/1890803002

Cr-Commit-Position: refs/heads/master@{#35793}