Re-apply "[JITLink] Update JITLink to use ExecutorAddr rather... " with fixes.

This re-applies 133f86e9, which was reverted in
c5965a41 while I investigated bot failures.

The original failure contained an arithmetic conversion think-o (on line 419 of
EHFrameSupport.cpp) that could cause failures on 32-bit platforms. The issue
should be fixed in this patch.