[MC] Use std::make_tuple to make some toolchains happy again

My toolchain (LLVM 8.0, libstdc++ 5.4.0) complained with:

12:27:43 ../lib/MC/MCDwarf.cpp:814:10: error: chosen constructor is explicit in copy-initialization
12:27:43   return {Offset, Size, SetDelta};
12:27:43          ^~~~~~~~~~~~~~~~~~~~~~~~
12:27:43 /proj/flexasic/app/llvm/8.0/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.4.0/../../../../include/c++/5.4.0/tuple:479:19: note: explicit constructor declared here
12:27:43         constexpr tuple(_UElements&&... __elements)
12:27:43                   ^
12:27:43 1 error generated.

This commit adds explicit calls to std::make_tuple to work around
the problem.