Reinstate `{Early,Late}LintPassObjects`.

Reinstate `{Early,Late}LintPassObjects`.

I removed these in #105291, and subsequently learned they are necessary
for performance.

This commit reinstates them with the new and more descriptive names
`RuntimeCombined{Early,Late}LintPass`, similar to the existing passes
like `BuiltinCombinedEarlyLintPass`. It also adds some comments,
particularly emphasising how we have ways to combine passes at both
compile-time and runtime. And it moves some comments around.