Sema: Look through FunctionConversionExpr in getEnclosingApplyExpr().

Follow up to #61481.

There is already a test case exercising this case but there happens to be no
observable difference in the compiler output because `noasync` diagnostics
already gracefully fall back to looking up the diagnostic `SourceLoc` another
way.

Resolves rdar://100883622.

Author: tshortli

lib/Sema/TypeCheckAvailability.cpp

@@ -3394,7 +3394,8 @@ class ExprAvailabilityWalker : public ASTWalker {
              isa<SelfApplyExpr>(parents[idx]) || // obj.f(a)
              isa<IdentityExpr>(parents[idx]) || // (f)(a)
              isa<ForceValueExpr>(parents[idx]) || // f!(a)
-             isa<BindOptionalExpr>(parents[idx])); // f?(a)
+             isa<BindOptionalExpr>(parents[idx]) || // f?(a)
+             isa<FunctionConversionExpr>(parents[idx]));
 
     auto *call = dyn_cast<ApplyExpr>(parents[idx]);
     if (!call || call->getFn() != parents[idx+1])