[TargetLowering] Correctly track NumFixedArgs field of CallLoweringInfo


[TargetLowering] Correctly track NumFixedArgs field of CallLoweringInfo

The NumFixedArgs field of CallLoweringInfo is used by
TargetLowering::LowerCallTo to determine whether a given argument is passed
using the vararg calling convention or not (specifically, to set IsFixed for
each ISD::OutputArg).

Firstly, CallLoweringInfo::setLibCallee and CallLoweringInfo::setCallee both
incorrectly set NumFixedArgs based on the _previous_ args list. Secondly,
TargetLowering::LowerCallTo failed to increment NumFixedArgs when modifying
the argument list so a pointer is passed for the return value.

If your backend uses the IsFixed property or directly accesses NumFixedArgs,
it is _possible_ this change could result in codegen changes (although the
previous behaviour would have been incorrect). No such cases have been
identified during code review for any in-tree architecture.

Differential Revision: https://reviews.llvm.org/D37898