Serialization: Update -experimental-skip-non-inlinable-function-bodies SIL verification for @_backDeploy.

SIL verification was failing for modules containing functions with `@_backDeploy` because `SILSkippingChecker` expected the `SILFunction` corresponding to the resilient copy of back deployed function to be empty. Since the overall function declaration for a back deployed function is considered inlinable, the body will be typechecked and SILGen emits both the fallback copy of the function and the resilient copy of the function. The checker should therefore expect to see back deployed functions that are not marked as serialized.

Author: tshortli

lib/SILOptimizer/UtilityPasses/SILSkippingChecker.cpp

@@ -72,6 +72,15 @@ static bool shouldHaveSkippedFunction(const SILFunction &F) {
       return false;
   }
 
+  // Functions with @_backDeploy may be copied into the client, so they
+  // shouldn't be skipped. The SILFunction that may be copied into the client
+  // should be serialized and therefore is already handled above. However, a
+  // second resilient SILFunction is also emitted for back deployed functions.
+  // Since the body of the function as written was not skipped, it's expected
+  // that we see the SILFunction for the resilient copy here.
+  if (func->isBackDeployed())
+    return false;
+
   // If none of those conditions trip, then this is something that _should_
   // be serialized in the module even when we're skipping non-inlinable
   // function bodies.

test/Serialization/back-deployed-attr-skip-noninlinable-function-bodies.swift

@@ -0,0 +1,5 @@
+// RUN: %target-swift-frontend -parse-as-library -enable-library-evolution -emit-module -module-name Test -experimental-skip-non-inlinable-function-bodies %s
+
+@available(SwiftStdlib 5.6, *)
+@_backDeploy(before: SwiftStdlib 5.7)
+public func foo() {}