Restore support for explicitly referenced CMakeLists.txt sources

Since commit v3.11.0-rc1~467^2 (VS,Xcode: Add CMakeLists.txt sources
without mutating targets, 2017-10-18) we do not add `CMakeLists.txt` to
target sources but instead generate references to them directly.  This
broke projects that explicitly specify their `CMakeLists.txt` file as a
source file because the explicit entry is no longer consolidated with
the generated one.

Teach the relevant generators to avoid duplicating `CMakeLists.txt`
source references and add test cases.

Fixes: #17828

Tests/RunCMake/XcodeProject/ExplicitCMakeLists-check.cmake

@@ -0,0 +1,20 @@
+set(xcProjectFile "${RunCMake_TEST_BINARY_DIR}/ExplicitCMakeLists.xcodeproj/project.pbxproj")
+if(NOT EXISTS "${xcProjectFile}")
+  set(RunCMake_TEST_FAILED "Project file ${xcProjectFile} does not exist.")
+  return()
+endif()
+
+set(foundCMakeLists 0)
+file(STRINGS "${xcProjectFile}" lines)
+foreach(line IN LISTS lines)
+  if(line MATCHES "PBXBuildFile.*fileRef.*CMakeLists.txt")
+    if(foundCMakeLists)
+      set(RunCMake_TEST_FAILED "CMakeLists.txt referenced multiple times")
+      return()
+    endif()
+    set(foundCMakeLists 1)
+  endif()
+endforeach()
+if(NOT foundCMakeLists)
+  set(RunCMake_TEST_FAILED "CMakeLists.txt not referenced")
+endif()

Tests/RunCMake/XcodeProject/ExplicitCMakeLists.cmake

@@ -0,0 +1,2 @@
+enable_language(CXX)
+add_executable(foo foo.cpp ${CMAKE_CURRENT_SOURCE_DIR}/CMakeLists.txt)

Tests/RunCMake/XcodeProject/RunCMakeTest.cmake

@@ -4,6 +4,8 @@ if(XCODE_VERSION VERSION_GREATER_EQUAL 9)
   set(IOS_DEPLOYMENT_TARGET "-DCMAKE_XCODE_ATTRIBUTE_IPHONEOS_DEPLOYMENT_TARGET=10")
 endif()
 
+run_cmake(ExplicitCMakeLists)
+
 run_cmake(XcodeFileType)
 run_cmake(XcodeAttributeLocation)
 run_cmake(XcodeAttributeGenex)