Inherit availability in @abi

ABI-only declarations now inherit `@available`, `@backDeployed`, etc. from their ABI counterpart. This will make it unnecessary to specify these attributes in `@abi`. Also some changes to make sure we suggest inserting `@available` in the correct place.

No tests because the enforcement is not yet in.

Author: beccadax

lib/AST/Availability.cpp

@@ -411,6 +411,11 @@ bool Decl::isAvailableAsSPI() const {
 
 SemanticAvailableAttributes
 Decl::getSemanticAvailableAttrs(bool includeInactive) const {
+  // A decl in an @abi gets its availability from the decl it's attached to.
+  auto abiRole = ABIRoleInfo(this);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart())
+    return abiRole.getCounterpart()->getSemanticAvailableAttrs(includeInactive);
+
   return SemanticAvailableAttributes(getAttrs(), this, includeInactive);
 }
 

lib/Sema/TypeCheckAttr.cpp

@@ -5138,7 +5138,8 @@ void AttributeChecker::checkBackDeployedAttrs(
     }
 
     if (auto *AFD = dyn_cast<AbstractFunctionDecl>(D)) {
-      if (!AFD->hasBody()) {
+      // Ignore this for ABI-only decls; ABIDeclChecker will diagnose it better.
+      if (!AFD->hasBody() && ABIRoleInfo(AFD).providesAPI()) {
         diagnoseAndRemoveAttr(Attr, diag::back_deployed_requires_body, Attr,
                               VD);
         continue;

lib/Sema/TypeCheckAvailability.cpp

@@ -480,6 +480,12 @@ static const Decl *relatedDeclForAvailabilityFixit(const Decl *D) {
     D = accessor->getStorage();
   }
 
+  auto abiRole = ABIRoleInfo(D);
+  if (!abiRole.providesAPI() && abiRole.getCounterpart()) {
+    // ABI-only decls can't have @available attributes of their own.
+    D = abiRole.getCounterpart();
+  }
+
   return D->getAbstractSyntaxDeclForAttributes();
 }
 
@@ -645,7 +651,7 @@ static void fixAvailabilityForDecl(
   // syntax to suggest the Fix-It may differ from the declaration to which
   // we attach availability attributes in the abstract syntax tree during
   // parsing.
-  const Decl *ConcDecl = D->getConcreteSyntaxDeclForAttributes();
+  const Decl *ConcDecl = relatedDeclForAvailabilityFixit(D);
 
   // To avoid exposing the pattern binding declaration to the user, get the
   // descriptive kind from one of the VarDecls.