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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276831 91177308-0d34-0410-b5e6-96231b3b80d8