Uses the base hashmap to store the ConstantArrayBuilder's constant map,
which slightly improves the performance of ConstantArrayBuilder::Insert.

Includes a small overload of the hashmap LookupOrInsert method, which
allows passing in a value creation function instead of just default
initialising new values.

On Octane's codeload, this gives (on my machine) a 0.27% improvement,
which doesn't sound like a lot but I guess every little helps.

Review-Url: https://codereview.chromium.org/2336553002
Cr-Commit-Position: refs/heads/master@{#39883}