Use variable tracking from ExpressionScopes rather than the PatternRewriter and
PreParserExpression::variables_ to declare variables.

We only figure out that variables are non-simple parameters once we see the
first non-simple parameter. This still uses the pattern rewriter to make
variables non-simple (kLet instead of kVar).

Change-Id: I4a4ee4852d667c26806bb24896722cfea3e093f2
Reviewed-on: https://chromium-review.googlesource.com/c/1417630Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58954}