[DWARFLinker] Convert analyzeContextInfo to a work list (NFC)

Convert analyzeContextInfo to a work list using the same approach I used
to remove the recursion from lookForDIEsToKeep. This fixes the crash
reported in https://llvm.org/PR48029.

Tested using the reproducer attached to PR48029 as well as by comparing
the clang MD5 hashes before and after the change (with and without
gmodules).

Differential revision: https://reviews.llvm.org/D90873