Add extern C markers

[brianmichel]

Ensure C linkage for shims to allow for usage in C++ interop environments.

Checklist

• [x] I've run tests to see all new and existing tests pass
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

If you've made changes to gyb files

• [x] I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request

Motivation:

This library when consumed from a swift package that has C++ interop enabled fails to successfully link since the symbols seem to get mangled.

Modifications:

I've added conditional extern "C" markers in the shims file if and only iff C++ is detected.

Result:

You should be able to consume and link this library in a target that has C++ interop enabled.

[brianmichel]

I feel uneasy about this. This only impacts the implementation and not the declaration which are consumed by the ClangImporter and thus this can lead to a mismatch in the linkage.

Ah this should likely just be in the header files, right? Sorry, long day of debugging.

[Lukasa]

@swift-server-bot test this please

[compnerd]

@Lukasa can we get this merged please?