[RuntimeDyld] Fix performance problem in resolveRelocations with many sections

Summary:
Rather than just iterating over all sections and checking whether we have relocations for them, iterate over the relocation map instead. This showed up heavily in an artificial julia benchmark that does lots of compilation. On that particular benchmark, this patch gives
~15% performance improvements. As far as I can tell the primary reason why the original
loop was so expensive is that Relocations[i] actually constructs a relocationList (allocating memory & doing lots of other unnecessary computing) if none is found.

Reviewers: lhames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13545

llvm-svn: 249942