[TBDGen] Fix a deserialization crash in TBDGen

Fix a deserialization failure crash in `TBDGenVisitor`, where an
internal protocol conformance with a project internal symbol witness for
an `associatedtype` triggers a crash inside `addConformances` because
the module containing the project internal symbol is not available.

Fixes rdar://93983322

Author: zixu-w

lib/TBDGen/TBDGen.cpp

@@ -479,6 +479,10 @@ void TBDGenVisitor::addConformances(const IterableDeclContext *IDC) {
   for (auto conformance : IDC->getLocalConformances(
                             ConformanceLookupKind::NonInherited)) {
     auto protocol = conformance->getProtocol();
+    if (Opts.PublicSymbolsOnly &&
+        getDeclLinkage(protocol) != FormalLinkage::PublicUnique)
+      continue;
+
     auto needsWTable =
         Lowering::TypeConverter::protocolRequiresWitnessTable(protocol);
     if (!needsWTable)

test/TBD/rdar93983322.swift

@@ -0,0 +1,44 @@
+// REQUIRES: OS=macosx
+// RUN: %empty-directory(%t)
+// RUN: mkdir -p %t/Modules
+// RUN: split-file %s %t
+
+// Build the project internal module
+// RUN: %target-swift-frontend %t/ProjectInternal.swift \
+// RUN:   -emit-module -module-name ProjectInternal \
+// RUN:   -o %t/Modules/ProjectInternal.swiftmodule \
+// RUN:   -disable-experimental-string-processing
+
+// Build MyModule with search path to the project internal module
+// RUN: %target-swift-frontend %t/MyModule.swift \
+// RUN:   -emit-module -module-name MyModule \
+// RUN:   -o %t/Modules/MyModule.swiftmodule \
+// RUN:   -I %t/Modules -disable-experimental-string-processing
+
+// Remove the project internal module as it's not available to clients of MyModule
+// RUN: rm %t/Modules/ProjectInternal.swiftmodule
+
+// Use swift-api-extract to load MyModule without ProjectInternal
+// RUN: %target-swift-api-extract -o - -pretty-print \
+// RUN:   -module-name MyModule -I %t/Modules
+
+//--- ProjectInternal.swift
+public class ProjectInternalClass {}
+
+//--- MyModule.swift
+@_implementationOnly import ProjectInternal
+
+public protocol PublicProtocol {}
+
+extension ProjectInternalClass : PublicProtocol {}
+
+internal protocol InternalProtocol {
+  associatedtype T : PublicProtocol
+  static var v : [T] { get }
+}
+
+public class PublicClass {}
+
+extension PublicClass : InternalProtocol {
+  static let v : [ProjectInternalClass] = []
+}