In switching to incremental parsing, we switched to pre-computing
line numbers for forward declarations. This is expensive,
because GetLineNumber reparses lines.

Also, switch typing maps to unordered_map (as they appear hot).

BUG=v8:4203
R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2553033002
Cr-Commit-Position: refs/heads/master@{#41525}