We cannot yet detect use-before-declaration in general, because for that we'd
need to analyze the context when compiling. But we can detect an error case
where we first see a use, then a declaration.

For this, I also added end position tracking (needed for error messages) to
VariableProxy.

Note: the position naming is completely inconsistent: start_position &
end_position, position & end_position, pos & end_pos, beg_pos & end_pos, to name
a few. This doesn't fix all of it, but tries to unify towards start_position &
end_position whenever possible w/ minimal changes.

BUG=

Review URL: https://codereview.chromium.org/943543002

Cr-Commit-Position: refs/heads/master@{#26880}