[Utils] Deal with occasionally deleted functions

When functions exist for some but not all run lines we need to be
careful when selecting the prefix. So far, a common prefix was
potentially chosen as there was never a "conflict" that would have
caused otherwise. With this patch we avoid common prefixes if they
are used by run lines that do not emit the function.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D68850