When Embedded Swift emits a symbol that was imported from another
module, ensure that the symbol is emitted as a weak definition. This
way, importing the same module (and using its symbol) into several
different modules doesn't cause duplicate-symbol errors at link time.
Rather, the linker will merge the different symbol definitions. This
makes Embedded Swift libraries work without resorting to
`-mergeable-symbols` or `-emit-empty-object-file`.
The test was crashing due to `swift_unreachable("custom executors not supported in embedded Swift")` line in `swift_task_enqueueImpl`, as the corresponding non-embedded codepath was relying on an unspecialized generic function `_swift_task_enqueueOnExecutor` defined in `Executor.swift`. Unspecialized generics are unavailable in Embedded Swift, and such `@silgen_name` function can't be specialized when used from concurrency runtime code written in C/C++. We can redefine this function for Embedded Swift as using a class-bound existential instead, and re-enable this codepath with a slightly different call that avoids the use of unavailable `swift_getObjectType` function from the non-embedded runtime.
The test is only partially enabled: we only check for IR correctness and correct linking. We don't run the tests on WASI yet due to unrelated issues that are going to be resolved separately.
