SectionMemoryManager shouldn't be a JITMemoryManager. Previously, the
EngineBuilder interface required a JITMemoryManager even if it was being used
to construct an MCJIT. But the MCJIT actually wants a RTDyldMemoryManager.
Consequently, the SectionMemoryManager, which is meant for MCJIT, derived
from the JITMemoryManager and then stubbed out a bunch of JITMemoryManager
methods that weren't relevant to the MCJIT.

This patch fixes the situation: it teaches the EngineBuilder that
RTDyldMemoryManager is a supertype of JITMemoryManager, and that it's
appropriate to pass a RTDyldMemoryManager instead of a JITMemoryManager if
we're using the MCJIT. This allows us to remove the stub methods from
SectionMemoryManager, and make SectionMemoryManager a direct subtype of
RTDyldMemoryManager.