Use std::make_tuple instead initializer list

Hopefully this pleases GCC-5 and fixes the build error:

LowerExpectIntrinsic.cpp:62:53: error: converting to
'std::tuple<unsigned int, unsigned int>' from initializer list would use
explicit constructor 'constexpr std::tuple<_T1, _T2>::tuple(_U1&&,
_U2&&) [with _U1 = llvm::cl::opt<unsigned int>&; _U2 =
llvm::cl::opt<unsigned int>&; <template-parameter-2-3> = void; _T1 =
unsigned int; _T2 = unsigned int]'
     return {LikelyBranchWeight, UnlikelyBranchWeight};

Differential Revision: https://reviews.llvm.org/D82325