Make compilers agree on source position of thrown errors.

This makes the compilers agree on the source position of a message
generated by "throw new Error()", it points to the beginning of the
throw directive.

R=titzer@chromium.org
TEST=message/regress/regress-3995
BUG=v8:3995
LOG=N

Review URL: https://codereview.chromium.org/1049703002

Cr-Commit-Position: refs/heads/master@{#27775}