Creates an AstStringConstants container which pre-initializes the
string constants used by AstValueFactory. This ensures that all
AstValueFactories will produce the same AstValue objects for constants,
and so they can be used by the BytecodeGenerator without having to pass
the AstValueFactory to it, enabling construction off-thread.

BUG=v8:5203

Review-Url: https://codereview.chromium.org/2630343002
Cr-Commit-Position: refs/heads/master@{#42381}