refactor(toolchain): fix --plugin-path on non-Xcode toolchains

[ma-oli]

Depending on a swift_compiler_plugin target currently adds:

--plugin-path
__BAZEL_XCODE_DEVELOPER_DIR__/usr/lib/swift/host/plugins/testing

to the compiler action. It's ok when using an Xcode toolchain, but when used with a standalone non-apple cc_toolchain (such as one from llvm_toolchain for instance), DEVELOPER_DIR will not be set, therefore causing the swift worker executable to either crash (on MacOS) or just return the empty string (on Linux). None of which are particularly helpful.

The right thing to do, from a swift perspective, would be to return a path inside the swift toolchain currently in use, regardless of whether we're using an Xcode toolchain or an alternative toolchain.

In this commit, we're therefore changing the meaning of the variable currently named __BAZEL_CUSTOM_XCODE_TOOLCHAIN_PATH__. It is currently meant to contain the name of the xcode toolchain when set through the TOOLCHAINS variable (in --action_env), and nothing otherwise. We're repurposing it to always return the swift toolchain location, regardless of the toolchain currently in use. When using TOOLCHAINS, it will return the path to the toolchain, and when using DEVELOPER_DIR, it will return the swift toolchain path inside the Xcode folder.

To reflect its new meaning, we're giving it a new name by the same occasion: BAZEL_SWIFT_TOOLCHAIN_PATH. Which just so happens, is also the private name it is given inside bazel_substitutions.cc.

Finally, we're making use of that new variable in
swift_compiler_plugin(), so --plugin-path work in non-Xcode environments.

[keith]

it's notable that the result here is every swift compile will now call xcrun in this where it didn't before