[RuntimeDyld] Speedup resolution of relocations to external symbols

From what I can tell, the loop inside applyExternalSymbolRelocations()
used to call getSymbolAddress(). After the JITSymbolResolver interface
redesign, the functionality has changed, and the loop should no longer
trigger repopulation of ExternalSymbolRelocations. If that's the case,
there is no need to update the loop iterator manually, and
ExternalSymbolRelocations can be cleared at once. This way, when there
are many external symbols in the program, the function runs much faster.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D97531