Optimize StrJoin with tuple without user defined formatter

[MBkkt]

#1671

Unfortunately the benchmarks can't be used with CMake and I don't have time to fix that or figure out how to use Bazel.

[derekmauro]

It may take me a few days to run some tests to make sure this doesn't break anything, but initially I'd say this is good work.

For reference, to build the benchmark:
bazel build -c opt absl/strings:str_join_benchmark

Then to run the benchmark:
bazel-bin/absl/strings/str_join_benchmark

Although it looks like the existing benchmark doesn't cover this case.