Embedded WASI: make `import _Concurrency` implicit (#88297)
**Explanation**: Existing `wasip1` triples for Embedded Swift already support concurrency, so this import should be implicit and no longer require package authors to make it explicit for Embedded Swift for WASI compatibility.
**Scope**: limited to Embedded Swift for Wasm;
**Risk**: low due to limited scope;
**Testing**: added a new lit test that verifies the change.
**Issue**: rdar://174021193
