Tests: Add mixed Swift+CXX source test case

This test ensures we can configure and build mixed source binaries. The
test configures, but fails to verify due to multiple targets emitting
the `lib.c.o` and `lib.cpp.o` outputs. Both the clang build step and the
swift link step report that they emit the `lib.c.o` and `lib.cpp.o`
outputs. The `.o`'s are emitted by clang, not by swift.

Tests/CMakeLists.txt

@@ -394,6 +394,9 @@ if(BUILD_TESTING)
     if(CMake_TEST_XCODE_SWIFT)
       ADD_TEST_MACRO(SwiftMix SwiftMix)
     endif()
+    if(CMAKE_Swift_COMPILER_VERSION VERSION_GREATER_EQUAL 5.1)
+      ADD_TEST_MACRO(SwiftMixLib Swifty)
+    endif()
   endif()
   if(CMAKE_Fortran_COMPILER)
     ADD_TEST_MACRO(FortranOnly FortranOnly)

Tests/SwiftMixLib/CMakeLists.txt

@@ -0,0 +1,6 @@
+cmake_minimum_required(VERSION 3.24)
+project(SwiftMixLib C CXX Swift)
+
+add_library(SwiftMixedLib lib.c lib.cpp lib.swift)
+add_executable(Swifty main.swift)
+target_link_libraries(Swifty PUBLIC SwiftMixedLib)

Tests/SwiftMixLib/lib.c

@@ -0,0 +1,4 @@
+int add_int(int a, int b)
+{
+  return a + b;
+}

Tests/SwiftMixLib/lib.cpp

@@ -0,0 +1,4 @@
+int add(int a, int b)
+{
+  return a + b;
+}

Tests/SwiftMixLib/lib.swift

@@ -0,0 +1,3 @@
+public func add(a: Int, b: Int) -> Int {
+    a + b
+}

Tests/SwiftMixLib/main.swift

@@ -0,0 +1,3 @@
+import SwiftMixedLib
+
+print(add(a: 1, b: 2))