of non-pattern expressions, according to the (internally circulated)
design document.  Details to be provided here.

1.  RewritableAssignmentExpression has been renamed to RewritableExpression.
    It is a wrapper for AST nodes that wait for some potential rewriting
    (that may or may not happen).  Also, Is... and As... macros now see
    through RewritableExpressions.

2.  The function state keeps a list of rewritable expressions that must be
    rewritten only if they are used as non-pattern expressions.

3.  Expression classifiers are now templates, parameterized by parser
    traits.  They keep some additional state: a pointer to the list of
    non-pattern rewritable expressions.  It is important that expression
    classifiers be used strictly in a stack fashion, from now on.

4.  The RewriteNonPattern function has been simplified.

BUG=chromium:579913
LOG=N

Review URL: https://codereview.chromium.org/1702063002

Cr-Commit-Position: refs/heads/master@{#34154}