[ORC-RT] Add non-const WrapperFunctionResult data access, simplify allocate.

WrapperFunctionResult no longer supports wrapping constant data, so this patch
provides direct non-const access to the wrapped data. Since wrapped data can now
be written, the WrapperFunctionResult::allocate method can be simplified to
return a WrapperFunctionResult.

This is essentially the same change (and with the same motivation) as LLVM
commit 8b117830, but applied to the ORC runtime's WrapperFunctionResult code.