We currently print asm.js functions converted to wasm as
"function foo() { [native code] }", even though without asm to wasm
translation we get the proper source code. This is an observable
difference that should not be, and also foozzie finds this frequently in
different variations.

This CL makes us remember the start position (position of the "function"
token) and end position (right behind the closing "}") of each function
we transform to wasm. These offsets, together with the Script that
contained the function, allows us to reconstruct the source code of the
function for the {toString()} method.

R=jkummerow@chromium.org

Bug: chromium:667678
Change-Id: If22471cad4cefdfc67f6d1b8fda85aa0eeb411bd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2016582
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65972}