[RuntimeDyld] Make RuntimeDyldImpl::resolveExternalSymbols preserve the
relocation entries it applies.

Prior to this patch, RuntimeDyldImpl::resolveExternalSymbols discarded
relocations for external symbols once they had been applied. This causes issues
if the client calls MCJIT::finalizeLoadedModules more than once, and updates the
location of any symbols in between (e.g. by calling MCJIT::mapSectionAddress).

No test case yet: None of our in-tree memory managers support moving sections
around. I'll have to hack up a dummy memory manager before I can write a unit
test.

Fixes <rdar://problem/16764378>