When parsing a eagerly-parsed-but-lazily-compiled function, we
used to put some of its AST nodes into a discardable Zone. This
CL puts the function Scope, its inner Scopes and the related AST
nodes (Declarations, VariableProxys) into the temporary Zone
too. This reduces peak memory usage and enables future work to
keep the temporary Zone around for later compilation.

BUG=

Review-Url: https://codereview.chromium.org/2193793002
Cr-Commit-Position: refs/heads/master@{#38232}