This reapplies4f0325873f(and follow up patches26fc07d5d8,a001cc0e6c,c9bc242e38, andfd174f0ff3), which were reverted in212cdc9a37to investigate bot failures (e.g. https://lab.llvm.org/buildbot/#/builders/108/builds/8502) The fix to address the bot failures was landed ind0052ebbe2. This patch also restricts construction of the UnwindInfoManager object to Apple platforms (as it won't be used on other platforms).