Remove PendingBody mechanism for function and ObjC method deserialization.

In its place, track on the canonical function declaration whether there is a
declaration with a body (and if so, which one). This brings function definition
handling in line with what we do in all other contexts, and is necessary to
allow us to merge declarations within multiple definitions of the same function
(eg, PR33924).

No functionality change intended.

llvm-svn: 314955