This makes strings_ and values_ in AstValueFactory a linked list through the AstString and AstValue objects. Additionally the CL computes whether strings are convertible to array indexes directly using the AstString's hash + literal bytes just as Name does, rather than indirecting over name if available.

BUG=

Review-Url: https://codereview.chromium.org/2225423002
Cr-Commit-Position: refs/heads/master@{#38536}