SILGen: Implement 'complete' unavailable decl optimization.

Avoid SIL lowering for declarations marked unavailable when
`-unavailable-decl-optimiation=complete` is specified.

Part of rdar://106674022

include/swift/SIL/SILModule.h

@@ -1078,8 +1078,22 @@ namespace Lowering {
 /// Determine whether the given class will be allocated/deallocated using the
 /// Objective-C runtime, i.e., +alloc and -dealloc.
 LLVM_LIBRARY_VISIBILITY bool usesObjCAllocator(ClassDecl *theClass);
+
+/// Returns true if SIL/IR lowering for the given declaration should be skipped.
+/// A declaration may not require lowering if, for example, it is annotated as
+/// unavailable and optimization settings allow it to be omitted.
+LLVM_LIBRARY_VISIBILITY bool shouldSkipLowering(Decl *D);
 } // namespace Lowering
 
+/// Apply the given function to each ABI member of \c D skipping the members
+/// that should be skipped according to \c shouldSkipLowering()
+template <typename F>
+void forEachMemberToLower(IterableDeclContext *D, F &&f) {
+  for (auto *member : D->getABIMembers()) {
+    if (!Lowering::shouldSkipLowering(member))
+      f(member);
+  }
+}
 } // namespace swift
 
 #endif