Follow-up build fix for rGae6f78824031

One of the bots objects to brace-initializing a tuple:

  http://lab.llvm.org:8011/builders/clang-cmake-x86_64-sde-avx512-linux/builds/43595/steps/build%20stage%201/logs/stdio

As the tuple constructor is apparently explicit. Fall back to the (not
as pretty) explicit construction of a tuple. I'd thought this was
permitted behaviour; will investigate why this fails later.