[MCJIT] Make repeat calls to MCJIT::getPointerToFunction for declarations safe.

MCJIT::getPointerForFunction adds the resulting address to the global mapping.
This should be done via updateGlobalMapping rather than addGlobalMapping, since
the latter asserts if a mapping already exists.

MCJIT::getPointerToFunction is actually deprecated - hopefully we can remove it
(or more likely re-task it) entirely soon. In the mean time it should at least
work as advertised.

<rdar://problem/18727946>