Follow up on https://github.com/protocolbuffers/protobuf/pull/6221

This avoids hitting the following warning when including GPBDescriptor.h in ARC
projects with stricter warning settings:
error: 'assign' property of object type may become a dangling reference; consider using 'unsafe_unretained' [-Werror,-Wobjc-property-assign-on-object-type

We first hit this in XCode 11, it's unclear whether it simply got stricter about
enforcing this warning or whether the flags enabled with -Wall or -Weverything
have been expanded.

In some platforms ${CMAKE_INSTALL_LIBDIR} expands to `lib64`. The libraries
are correctly installed in that directory, but the RPATH is set to point to
`$ORIGIN/../lib`, where it should be `$ORIGIN/../lib64`.

Merge 3.8.x into master branch

The wire_format_lite_inl.h header file was made unnecessary in #5839
and deleted in #5921, so remove the comment about it.

Update CONTRIBUTING.md to clarify who should set the labels

Down integrate to GitHub

release notes: no

I'm getting `src/google/protobuf/map_field.cc(284): error C2899: typename cannot be used outside a template declaration`

removing `typename` fixes the issue

This fixes compatibility with bazel when passing `--incompatible_no_support_tools_in_action_inputs` which will be flipped in an upcoming release.

Update protobuf version to 3.8.0

Fix gulpfile to use gulp 4 API

Previously, foo_value and foo (wrapper field) will both have get/setFooValue

Down integrate to GitHub

Cherrypick 014872a9 to fix build

This reverts commit a7180caf.

This reverts commit 783971e8.
Revert an accidental revert!

This reverts commit 6d96493b.

This reverts commit c40018e5.

Update performance.md

Even though I just proofread this file, I noticed one more thing that needed correcting, which I fixed. The word "genereated" had to be corrected to "generated". My apologies for missing this on the initial check.

Down Integrate to GitHub

Update performance.md

I corrected a few grammatical mistakes and increased the document's readability by easing the language flow.