Make TemplateLiteral hashing algorithm more memory efficient

Previously, a separate string to be hashed (in order to help determine the need to
use a cached Template Call Site) was built up by joining UTF8 spans within a template.

Now, the hash key is generated from the original spans, removing the need to allocate a new
buffer and copy bytes into it.

BUG=

Review URL: https://codereview.chromium.org/765473006

Cr-Commit-Position: refs/heads/master@{#25549}