Parsing especially large nested functions takes up more memory than necessary.

Parsing especially large nested functions takes up more memory than necessary.
Inner functions must be eagerly parsed for scope analysis, but the full AST is
also kept around even though it's not needed.

This CL mitigates this problem by allocating some AstNodes of the inner function
to a temporary Zone which is deallocated once the scope information has been
built. The remaining nodes (such as VariableProxy) must persist until scope
analysis actually happens, and have to be allocated to a parser-persistent Zone.

BUG=417697
LOG=N

Review URL: https://codereview.chromium.org/1304923004

Cr-Commit-Position: refs/heads/master@{#30685}