[Utils][FIX] Properly deal with occasionally deleted functions

While D68850 allowed functions to be deleted I accidentally saved some
version of the function to be used once a suitable prefix was found.
This turned out to be problematic when the occasionally deleted function
is also occasionally modified. The test case is adjusted to resemble the
case in which the problem was found.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D76586