Don't use "return {...}" to initialize a std::tuple. This has only been valid

Don't use "return {...}" to initialize a std::tuple. This has only been valid
since 2015 (n4387), though it's allowed by a library DR so new implementations
accept it in their C++11 modes...

This should unbreak the build with libstdc++ 4.9.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279583 91177308-0d34-0410-b5e6-96231b3b80d8