IRGen: Use CallEmission to emit dispatch thunks

This increases the level of abstraction a bit and makes it easier to stage
in the requisite support for async method calls. For now, I've kept the
existing, incomplete logic for those.

Part of rdar://problem/73625623.

Author: slavapestov

Diff for: lib/IRGen/CallEmission.h

@@ -103,6 +103,13 @@ class CallEmission {
                     bool isOutlined);
   void emitToExplosion(Explosion &out, bool isOutlined);
 
+  llvm::CallInst *emitCoroutineAsOrdinaryFunction() {
+    assert(IsCoroutine);
+    IsCoroutine = false;
+
+    return emitCallSite();
+  }
+
   TemporarySet claimTemporaries() {
     // Move the actual temporary set out.
     auto result = std::move(Temporaries);