fix explicit constructor in copy-initialization

[Mythra]

Description:

commit: 4c80194#diff-5d52e6425da0cdb53f84da8906c0a968R602
happened to break our builds because of the use of return {}'s
it introduced into upstream_impl.cc (we use a toolchain built around
clang-9, to more accurately map with the rest of our tools). this
fixes that build error.

Risk Level: Low
Testing:

Testing is hard in this one since as far as I can tell it doesn't happen with the toolchain inside of envoy-build-ubuntu. However, I scrapped together a quick public image you can use to validate:

1. Checkout master of envoy
2. Run the following:

docker pull gcr.io/ecoan-1280/ggg-bazel-image:latest
docker run \
       -e "CC=clang" \
       -e "CXX=clang++" \
       -it --rm \
 --net=host \
       --cap-add SYS_PTRACE \
       -v "$(pwd)":"/mnt/envoy" \
       -w "/mnt/envoy" \
       "gcr.io/ecoan-1280/ggg-bazel-image:latest" \
       /bin/bash
$ bazel build //source/exe:envoy-static

3. See build errors like:

source/common/upstream/upstream_impl.cc:735:10: error: chosen constructor is explicit in copy-initialization
  return {std::move(filtered_clones[0]), std::move(filtered_clones[1]),
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

4. Checkout this commit into your branch.
5. Re-run the docker steps again.
6. Watch the build complete successfully.

Docs Changes: N/A
Release Notes: N/A

Signed-off-by: Eric ecoan@rust-lang.life