Swift: Fix swiftmodule dependency

Fix a bug in the split build model where we generate a dependency on the
swiftmodule of a target that does not emit a swiftmodule.

This fix drops the erroneous swiftmodule dependency edge, avoiding the
broken dependency graph. Incremental builds still work with the new
model because of the swiftmodule dependency edges between Swift
compilation steps.

Fixes: #26922

Source/cmNinjaNormalTargetGenerator.cxx

@@ -1578,7 +1578,8 @@ void cmNinjaNormalTargetGenerator::WriteLinkStatement(
   }
 
   // Add dependencies on swiftmodule files when using the swift linker
-  if (this->TargetLinkLanguage(config) == "Swift") {
+  if (!this->GetLocalGenerator()->IsSplitSwiftBuild() &&
+      this->TargetLinkLanguage(config) == "Swift") {
     if (cmComputeLinkInformation* cli =
           this->GeneratorTarget->GetLinkInformation(config)) {
       for (auto const& dependency : cli->GetItems()) {

Tests/RunCMake/Swift/MultiLangChain.cmake

@@ -0,0 +1,10 @@
+cmake_policy(SET CMP0157 NEW)
+cmake_policy(SET CMP0195 NEW)
+
+enable_language(Swift C)
+
+add_library(L L.swift)
+add_library(C C.c)
+target_link_libraries(C PRIVATE L)
+add_library(E E.swift)
+target_link_libraries(E PRIVATE C)

Tests/RunCMake/Swift/RunCMakeTest.cmake

@@ -30,6 +30,17 @@ block()
   endif()
 endblock()
 
+# Older Xcode versions didn't support Swift static libraries.
+if(NOT (RunCMake_GENERATOR STREQUAL "Xcode" AND XCODE_VERSION VERSION_LESS 9.0))
+  block()
+    set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/MultiLangChain-build)
+    run_cmake(MultiLangChain)
+    set(RunCMake_TEST_NO_CLEAN 1)
+    set(RunCMake_TEST_OUTPUT_MERGE 1)
+    run_cmake_command(MultiLangChain-build ${CMAKE_COMMAND} --build .)
+  endblock()
+endif()
+
 if(RunCMake_GENERATOR MATCHES "Ninja.*")
   block()
     set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/CMP0195-NEW-build)