[SILGen] fix crash invoking global-actor qualified optional ObjC async method

Due to some changes in how the AST is constructed by CSApply to handle
global actors from @preconcurrency declarations, the AST for a force-call to
an optional ObjC method that is part of a global-actor qualified protocol, such as:

```
import Foundation

@mainactor
@objc protocol P {
  @objc optional func generateMaybe() async
}

extension P {
  func test() async -> Void {
        await self.generateMaybe!()
    }
}
```

now contains a function conversion in between the forced-value operation and the dynamic-ref:

```
(force_value_expr type='@mainactor () async -> ()'
  (optional_evaluation_expr implicit type='(@mainactor () async -> ())?'
    (inject_into_optional implicit type='(@mainactor () async -> ())?'
      (function_conversion_expr implicit type='@mainactor () async -> ()'   <<<<< new!
        (bind_optional_expr implicit type='() async -> ()'
          (dynamic_member_ref_expr type='(() async -> ())?' ... )))))
```

For compatibility with how ObjC does message sends to these optional methods, in Swift there
is a difference between how something like `a.method!()` and the following are compiled:

```
let meth = a.method!
meth()
```

The former will directly call the optional method and let the "unknown selector" error be emitted
if it's not implemented. But the latter will query the dynamic method and inject that result into an
`Optional<...>` to then be forced, yielding a "force-unwrap nil" error. It's a subtle difference but
those two scenarios lead to different code paths in SILGen.

Now, this patch fixes the issue by enhancing the recognition of these direct call scenarios so that it
can look past these function conversions. Because that recognition already tries to ignore implicit
conversions, this is not something new. The problem was that we weren't considering implicit conversions
between the optional-evaluation and bind-optional expressions.

This patch is intentionally precise about what kind of function conversions can be skipped, because I
do not know if all of them are OK to blindly skip or not. We might need to expand that in the future.

Resolves rdar://97646309

Author: kavon

lib/SILGen/SILGenApply.cpp

@@ -1592,6 +1592,39 @@ class SILGenApply : public Lowering::ExprVisitor<SILGenApply> {
     return std::move(*applyCallee);
   }
 
+  /// \returns true if the conversion is from an async function to
+  /// the same type but with a global actor added. For example this:
+  ///     () async -> ()  ==>  @MainActor () async -> ()
+  /// will return true. In all other cases, returns false.
+  static bool addsGlobalActorToAsyncFn(FunctionConversionExpr *fce) {
+    CanType oldTy = fce->getSubExpr()->getType()->getCanonicalType();
+    CanType newTy = fce->getType()->getCanonicalType();
+
+    if (auto oldFnTy = dyn_cast<AnyFunctionType>(oldTy)) {
+      if (auto newFnTy = dyn_cast<AnyFunctionType>(newTy)) {
+        // old type MUST be async
+        if (!oldFnTy->hasEffect(EffectKind::Async))
+          return false;
+
+        // old type MUST NOT have a global actor
+        if (oldFnTy->hasGlobalActor())
+          return false;
+
+        // new type MUST have a global actor
+        if (!newFnTy->hasGlobalActor())
+          return false;
+
+        // see if adding the global actor to the old type yields the new type.
+        auto globalActor = newFnTy->getGlobalActor();
+        auto addedActor = oldFnTy->getExtInfo().withGlobalActor(globalActor);
+
+        return oldFnTy->withExtInfo(addedActor) == newFnTy;
+      }
+    }
+
+    return false;
+  }
+
   /// Ignore parentheses and implicit conversions.
   static Expr *ignoreParensAndImpConversions(Expr *expr) {
     while (true) {
@@ -1605,7 +1638,16 @@ class SILGenApply : public Lowering::ExprVisitor<SILGenApply> {
       // works given that we check the result for certain forms.
       if (auto eval = dyn_cast<OptionalEvaluationExpr>(expr)) {
         if (auto inject = dyn_cast<InjectIntoOptionalExpr>(eval->getSubExpr())) {
-          if (auto bind = dyn_cast<BindOptionalExpr>(inject->getSubExpr())) {
+
+          auto nextSubExpr = inject->getSubExpr();
+
+          // skip over a specific, known no-op function conversion, if it exists
+          if (auto funcConv = dyn_cast<FunctionConversionExpr>(nextSubExpr)) {
+            if (addsGlobalActorToAsyncFn(funcConv))
+              nextSubExpr = funcConv->getSubExpr();
+          }
+
+          if (auto bind = dyn_cast<BindOptionalExpr>(nextSubExpr)) {
             if (bind->getDepth() == 0)
               return bind->getSubExpr();
           }