Merge pull request apple#1515 from thomasvl/version_bump_1_x

Version bump to do a release with the messageset fix. (1.x branch)

Revision bump to do an update release for the xcode project file.

Update the generated_swift_names_* files on the 1.x branch.

Version bump to ready a release.

1.x version bump for a release.

Version bump for a release.

There are no library changes, just the new plugin and the new error reporting
for the plugin.

Exposes implementationOnlyImports option in SPM plugin

# Conflicts:
#	Sources/protoc-gen-swift/Docs.docc/index.md

1.x branch: bump to 1.21.0 to make a release.

Version bump for a release.

Bump to 1.20.2 for a release.