Re-apply fixed r354847

unifdef(1): Improve worst-case bound on symbol resolution

Use RB_TREE to make some algorithms O(lg N) and O(N lg N) instead of O(N)
and O(N^2).

While here, remove arbitrarily limit on number of macros understood.

Reverts r354877 and r354878, which disabled the (correct) test.

PR: 242095
Reported by: lwhsu