AstValueFactory: make true, false, null, undefined and "the hole" unique values.

They were not, so we were creating several instances of them, one for each time
they occurred in the source code.

It's not known to have caused efficiency problems though, so this is a sanity
fix more than an efficiency fix.

Note that numbers are still not unique.

BUG=
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/657893003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24873 ce2b1a6d-e550-0410-aec6-3dcde31c8c00