[llvm-cov gcov] Make recursive propagateCounts iterative. NFC

propagateCounts computes unmeasured arc counts (see
commit b9d08669).

In a x86-64 build using -O3 -fno-omit-frame-pointer, propagateCounts uses 80
bytes per stack frame. If a function contains 1e5 basic blocks on a tree path
(Kirchoff's circuit law optimization), the used stack space will be 8MB (default
ulimit -s in many configurations). (In a -O0 build, a stack frame costs 224
bytes.) 1e5 is ample for most configurations. However, for library users using
threads (e.g. in RPC handlers), a remaining thread stack of 64KiB allows just
819 stack frames, which is too limited.

Switch to an iterative form to avoid stack overflow issues. Iterative forms
match other iterative form functions in this file
(https://reviews.llvm.org/D93073).

Alternative to #68455