Swift: honour `-swift-version` in Ninja generator

Swift has supported `CMAKE_Swift_LANGUAGE_VERSION` and
`Swift_LANGUAGE_VERSION` but didn't apply that to Ninja generated
targets.  Consider the property when calculating the flags.

Help/manual/cmake-properties.7.rst

@@ -316,6 +316,7 @@ Properties on Targets
    /prop_tgt/STATIC_LIBRARY_OPTIONS
    /prop_tgt/SUFFIX
    /prop_tgt/Swift_DEPENDENCIES_FILE
+   /prop_tgt/Swift_LANGUAGE_VERSION
    /prop_tgt/Swift_MODULE_DIRECTORY
    /prop_tgt/Swift_MODULE_NAME
    /prop_tgt/TYPE

Help/prop_tgt/Swift_LANGUAGE_VERSION.rst

@@ -0,0 +1,6 @@
+Swift_LANGUAGE_VERSION
+----------------------
+
+This property sets the language version for the Swift sources in the target.  If
+one is not specified, it will default to ``<CMAKE_Swift_LANGUAGE_VERSION>`` if
+specified, otherwise it is the latest version supported by the compiler.

Source/cmLocalGenerator.cxx

@@ -1566,6 +1566,17 @@ void cmLocalGenerator::AddLanguageFlags(std::string& flags,
   this->AddConfigVariableFlags(flags, cmStrCat("CMAKE_", lang, "_FLAGS"),
                                config);
 
+  if (lang == "Swift") {
+    if (const char* v = target->GetProperty("Swift_LANGUAGE_VERSION")) {
+      if (cmSystemTools::VersionCompare(
+            cmSystemTools::OP_GREATER_EQUAL,
+            this->Makefile->GetDefinition("CMAKE_Swift_COMPILER_VERSION"),
+            "4.2")) {
+        this->AppendFlags(flags, "-swift-version " + std::string(v));
+      }
+    }
+  }
+
   // Add MSVC runtime library flags.  This is activated by the presence
   // of a default selection whether or not it is overridden by a property.
   const char* msvcRuntimeLibraryDefault =

Source/cmTarget.cxx

@@ -347,6 +347,7 @@ cmTarget::cmTarget(std::string const& name, cmStateEnums::TargetType type,
     initProp("LINK_SEARCH_START_STATIC");
     initProp("LINK_SEARCH_END_STATIC");
     initProp("FOLDER");
+    initProp("Swift_LANGUAGE_VERSION");
     initProp("Swift_MODULE_DIRECTORY");
     initProp("VS_JUST_MY_CODE_DEBUGGING");
 #ifdef __APPLE__