"this" is a very common expression. By using a single ThisExpression object
we can both avoid allocating many unnecessary VariableProxies and specialize
the resolution of this since we know where it's declared up-front. This also
avoids having to special-case "this" reference handling in the paths that would
behave differently for "this" than for regular references; e.g., with-scopes.

The tricky pieces are due to DebugEvaluate and this/super() used as default
parameters of arrow functions. In the former case we replace the WITH_SCOPE
with FUNCTION_SCOPE so that we make sure that "this" is intercepted, and still
rely on regular dynamic variable lookup. Arrow functions are dealt with by
marking "this" use in ArrowHeadParsingScopes. If the parenthesized expression
ends up being an arrow function, we force context allocate on the outer scope
(and mark "has_this_reference" on the FUNCTION_SCOPE so DebugEvaluate in the
arrow function can expose "this").

The CL also removes the now unused ThisFunction AST node.

Change-Id: I0ca38ab92ff58c2f731e07db2fbe91df901681ef
Reviewed-on: https://chromium-review.googlesource.com/c/1448313Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59393}