This adds support for tracking token positions in the asm.js scanner and
uses these positions to emit a mapping from WASM to asm.js positions.
Note that the mapping is still incomplete (some call sites are not yet
covered).

R=clemensh@chromium.org
TEST=debugger/debug/wasm/asm-debug
BUG=v8:6127

Change-Id: Ic8aad1a85e7d9e19da2eec523fcc73d4984afcc8
Reviewed-on: https://chromium-review.googlesource.com/466046
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44335}