ABIChecker: exclude decls with the @_alwaysEmitIntoClient attribute

Removing or updating @_alwaysEmitIntoClient functions are never ABI-breaking, thus
we should exclude them from the symbol set for ABI stability checking.

rdar://67883661

Author: nkcsgexi

test/api-digester/Inputs/cake.swift

@@ -139,3 +139,6 @@ public class UnavailableOnMac {}
 extension SwiftObjcClass {
   public func functionUnavailableOnMac() {}
 }
+
+@_alwaysEmitIntoClient
+public func emitIntoClientFunc() {}

test/api-digester/Outputs/cake.json

@@ -1340,6 +1340,25 @@
       ],
       "hasMissingDesignatedInitializers": true
     },
+    {
+      "kind": "Function",
+      "name": "emitIntoClientFunc",
+      "printedName": "emitIntoClientFunc()",
+      "children": [
+        {
+          "kind": "TypeNominal",
+          "name": "Void",
+          "printedName": "()"
+        }
+      ],
+      "declKind": "Func",
+      "usr": "s:4cake18emitIntoClientFuncyyF",
+      "moduleName": "cake",
+      "declAttributes": [
+        "AlwaysEmitIntoClient"
+      ],
+      "funcSelfKind": "NonMutating"
+    },
     {
       "kind": "TypeDecl",
       "name": "Int",

tools/swift-api-digester/ModuleAnalyzerNodes.cpp

@@ -1603,6 +1603,11 @@ SDKContext::shouldIgnore(Decl *D, const Decl* Parent) const {
       if (isa<TypeAliasDecl>(VD))
         return true;
     }
+    // Exclude decls with @_alwaysEmitIntoClient if we are checking ABI.
+    // These decls are considered effectively public because they are usable
+    // from inline, so we have to manually exclude them here.
+    if (D->getAttrs().hasAttribute<AlwaysEmitIntoClientAttr>())
+      return true;
   } else {
     if (D->isPrivateStdlibDecl(false))
       return true;

tools/swift-api-digester/swift-api-digester.cpp

@@ -1057,6 +1057,17 @@ void swift::ide::api::SDKNodeTypeFunc::diagnose(SDKNode *Right) {
 }
 
 namespace {
+static void diagnoseRemovedDecl(const SDKNodeDecl *D) {
+  if (D->getSDKContext().checkingABI()) {
+    // Don't complain about removing @_alwaysEmitIntoClient if we are checking ABI.
+    // We shouldn't include these decls in the ABI baseline file. This line is
+    // added so the checker is backward compatible.
+    if (D->hasDeclAttribute(DeclAttrKind::DAK_AlwaysEmitIntoClient))
+      return;
+  }
+  D->emitDiag(SourceLoc(), diag::removed_decl, D->isDeprecated());
+}
+
 // This is first pass on two given SDKNode trees. This pass removes the common part
 // of two versions of SDK, leaving only the changed part.
 class PrunePass : public MatchedNodeListener, public SDKTreeDiffPass {
@@ -1241,7 +1252,7 @@ class PrunePass : public MatchedNodeListener, public SDKTreeDiffPass {
                      TD->isProtocol());
       }
       if (auto *Acc = dyn_cast<SDKNodeDeclAccessor>(Left)) {
-        Acc->emitDiag(SourceLoc(), diag::removed_decl, Acc->isDeprecated());
+        diagnoseRemovedDecl(Acc);
       }
       return;
     case NodeMatchReason::FuncToProperty:
@@ -2084,7 +2095,7 @@ static bool diagnoseRemovedExtensionMembers(const SDKNode *Node) {
     if (DT->isExtension()) {
       for (auto *C: DT->getChildren()) {
         auto *MD = cast<SDKNodeDecl>(C);
-        MD->emitDiag(SourceLoc(), diag::removed_decl, MD->isDeprecated());
+        diagnoseRemovedDecl(MD);
       }
       return true;
     }
@@ -2161,7 +2172,7 @@ void DiagnosisEmitter::handle(const SDKNodeDecl *Node, NodeAnnotation Anno) {
     }
     bool handled = diagnoseRemovedExtensionMembers(Node);
     if (!handled)
-      Node->emitDiag(SourceLoc(), diag::removed_decl, Node->isDeprecated());
+      diagnoseRemovedDecl(Node);
     return;
   }
   case NodeAnnotation::Rename: {