IRGen: keep emitting protocol witness table symbols for refactored protocols

When Protocol P and Struct S are in a same module and S conforms to P, the protocol
witness table is emitted directly as a symbol. If we move P to a lower-level module,
the protocol witness table symbol isn't emitted, breaking users' existing executable as
a result.

This change checks whether the protocol used to be defined in the same
module and marks it as non-resilient if so. The compiler will continue
emitting witness table as symbols to maintain cross-module ABI stability.

Author: nkcsgexi

lib/AST/Decl.cpp

@@ -3535,7 +3535,11 @@ bool NominalTypeDecl::isResilient(ModuleDecl *M,
   case ResilienceExpansion::Minimal:
     return isResilient();
   case ResilienceExpansion::Maximal:
-    return M != getModuleContext() && isResilient();
+    // We consider this decl belongs to the module either it's currently
+    // defined in this module or it's originally defined in this module, which
+    // is specified by @_originallyDefinedIn
+    return M != getModuleContext() &&
+      M->getName().str() != getAlternateModuleName() && isResilient();
   }
   llvm_unreachable("bad resilience expansion");
 }

lib/IRGen/GenProto.cpp

@@ -828,6 +828,15 @@ bool IRGenModule::isResilientConformance(
       conformanceModule == conformance->getProtocol()->getParentModule())
     return false;
 
+  // If the protocol WAS from the current module (@_originallyDefinedIn), we
+  // consider the conformance non-resilient, because we used to consider it
+  // non-resilient before the symbol moved. This is to ensure ABI stability
+  // across module boundaries.
+  if (conformanceModule == getSwiftModule() &&
+      conformanceModule->getName().str() ==
+        conformance->getProtocol()->getAlternateModuleName())
+    return false;
+
   // If the protocol and the conformance are in the same module and the
   // conforming type is not generic, they're not resilient.
   //

test/attr/attr_originally_definedin_backward_compatibility.swift

@@ -11,7 +11,7 @@
 // RUN: mkdir -p %t/SDK/Frameworks/HighLevel.framework/Modules/HighLevel.swiftmodule
 // RUN: %target-build-swift-dylib(%t/SDK/Frameworks/HighLevel.framework/HighLevel) -module-name HighLevel -emit-module \
 // RUN:		-emit-module-path %t/SDK/Frameworks/HighLevel.framework/Modules/HighLevel.swiftmodule/%module-target-triple.swiftmodule \
-// RUN:     %S/Inputs/SymbolMove/HighLevelOriginal.swift -Xlinker -install_name -Xlinker @rpath/HighLevel.framework/HighLevel
+// RUN:     %S/Inputs/SymbolMove/HighLevelOriginal.swift -Xlinker -install_name -Xlinker @rpath/HighLevel.framework/HighLevel -enable-library-evolution
 
 // --- Build an executable using the original high level framework
 // RUN: %target-build-swift -emit-executable %s -g -o %t/HighlevelRunner -F %t/SDK/Frameworks/ -framework HighLevel \
@@ -24,13 +24,13 @@
 // RUN: mkdir -p %t/SDK/Frameworks/LowLevel.framework/Modules/LowLevel.swiftmodule
 // RUN: %target-build-swift-dylib(%t/SDK/Frameworks/LowLevel.framework/LowLevel) -module-name LowLevel -emit-module \
 // RUN:		-emit-module-path %t/SDK/Frameworks/LowLevel.framework/Modules/LowLevel.swiftmodule/%module-target-triple.swiftmodule \
-// RUN:     %S/Inputs/SymbolMove/LowLevel.swift -Xlinker -install_name -Xlinker @rpath/LowLevel.framework/LowLevel
+// RUN:     %S/Inputs/SymbolMove/LowLevel.swift -Xlinker -install_name -Xlinker @rpath/LowLevel.framework/LowLevel -enable-library-evolution
 
 // --- Build high level framework.
 // RUN: mkdir -p %t/SDK/Frameworks/HighLevel.framework/Modules/HighLevel.swiftmodule
 // RUN: %target-build-swift-dylib(%t/SDK/Frameworks/HighLevel.framework/HighLevel) -module-name HighLevel -emit-module \
 // RUN:		-emit-module-path %t/SDK/Frameworks/HighLevel.framework/Modules/HighLevel.swiftmodule/%module-target-triple.swiftmodule \
-// RUN:     %S/Inputs/SymbolMove/HighLevel.swift -F %t/SDK/Frameworks -Xlinker -reexport_framework -Xlinker LowLevel
+// RUN:     %S/Inputs/SymbolMove/HighLevel.swift -F %t/SDK/Frameworks -Xlinker -reexport_framework -Xlinker LowLevel -enable-library-evolution
 
 // --- Run the executable
 // RUN: %t/HighlevelRunner | %FileCheck %s -check-prefix=AFTER_MOVE