Eliminate extra set of simd variant function attribute.

Current clang generates extra set of simd variant function attribute
with extra 'v' encoding.
For example:
_ZGVbN2v__Z5add_1Pf vs _ZGVbN2vv__Z5add_1Pf
The problem is due to declaration of ParamAttrs following:
    llvm::SmallVector<ParamAttrTy, 8> ParamAttrs(ParamPositions.size());
where ParamPositions.size() is grown after following assignment:
    Pos = ParamPositions[PVD];
So the PVD is not find in ParamPositions.

The problem is ParamPositions need to set for each FD decl. To fix this

Move ParamPositions's init inside while loop for each FD.

Differential Revision: https://reviews.llvm.org/D122338