[Debugify] Limit number of processed functions for original mode

Debugify in OriginalDebugInfo mode, does (DebugInfo) collect-before-pass & check-after-pass
for each instruction, which is pretty expensive. When used to analyze DebugInfo losses
in large projects (like LLVM), this raises the build time unacceptably.
This patch introduces a limit for the number of processed functions per compile unit.
By default, the limit is set to UINT_MAX (practically unlimited), and by using the introduced
option  -debugify-func-limit  the limit could be set to any positive integer number.

Differential revision: https://reviews.llvm.org/D115714