[ORC] Keep weak flag on VSO symbol tables during materialization, but treat

materializing weak symbols as strong.

This removes some elaborate flag tweaking and plays nicer with RuntimeDyld,
which relies of weak/common flags to determine whether it should emit a given
weak definition. (Switching to strong up-front makes it appear as if there is
already an overriding definition, which would require an extra back-channel to
override).

llvm-svn: 334966