Allow the Swift toolchain to be associated with an execution group and pass an optional execution group name down to actions that use it

[brentleyjones]

This updates the following APIs:

• swift_common.get_toolchain now takes an exec_group argument, which is will look up the toolchain under ctx.exec_groups[NAME] instead of ctx.
• swift_common.{compile,compile_module_interface,precompile_clang_module} now take an exec_group argument, which is passed down to the underlying ctx.actions.run call for actions using the toolchain.

In all cases the default value of the argument is None, so omitting it retains the current behavior.

Most importantly, this lets us declare the Swift toolchain somewhere other than the rule/aspect's ctx, which ensures that in a multiple toolchain scenario, the Swift toolchain doesn't fully decide the execution platform for all actions (even non-Swift actions).

Since execution groups are somewhat in flux right now, this may not be the final form—it would be nice to have the coupling between the execution group and the toolchain automated. I considered just having each action be its own execution group (like C++ link actions use cpp_link), but that seems worse; Swift actions should almost never be split across different toolchains/platforms, and it would force the user to have to declare multiple exec groups if they wanted to use the feature with multiple actions. This lets them share something like a single "swift" execution group across multiple actions, with the only caveat being that they have to pass that name explicitly.

PiperOrigin-RevId: 502638394
(cherry picked from commit aeee2bb)