Fix for compiling with clang <= 3.7 and g++6 headers

Make integers explicitly unsigned, so the tuple constructor will resolve
properly when but with clang 3.6, 3.7 and gcc 6.1.1 libstdc++ headers.

Patch by Frederich Munch!

Differential Revision: https://reviews.llvm.org/D22798

llvm-svn: 276831