Add an option to MCJIT to have it forward all sections to the
RTDyldMemoryManager, regardless of whether it thinks they're "required for
execution".

Currently, RuntimeDyld only passes sections that are "required for execution"
to the RTDyldMemoryManager, and takes "required for execution" to mean exactly
"contains symbols or relocations". There are two problems with this:
(1) It can drop sections with anonymous data that is referenced by code.
(2) It leaves the JIT client no way to inspect interesting sections that aren't
    actually required to run the program (e.g dwarf sections).

A test case is still in the works.

Future work: We may want to replace this with a generic section filtering
mechanism, but that will require more consideration. For now, this flag at least
allows clients to volunteer to do the filtering themselves.

Fixes <rdar://problem/15177691>.