[BOLT] Speedup RTDyld external symbol resolution

Summary:
RuntimeDyldImpl::resolveExternalSymbols() some time ago used to call
getSymbolAddress() while in the second loop. That call could have
modified the contents of ExternalSymbolRelocations that the loop was
iterating over. Thus the code was written in a way that erased the
processed entry on every loop iteration and reset the map
iterator. With large number of entries in ExternalSymbolRelocations the
loop code becomes a performance bottleneck.

Since getSymbolAddress() is no longer used, the
ExternalSymbolRelocations could be iterated in a straightforward way and
the map cleared before the function exit.

(cherry picked from FBD21057058)