cmGlobalGenerator: Allow passing language to GetLangaugeOutputExtension

The original GetLanguageOutputExtension took a sourcefile instead of the
name of the language itself. This implementation provided a convenient
handler for when the SourceFile doesn't know what language it is, but
there are times where we know the language, but don't necessarily have a
source file. Adding an overload that takes the name of the language and
returns the extension of that language, or empty string if no extension
is registered.

Source/cmGlobalGenerator.cxx

@@ -1158,24 +1158,30 @@ std::string cmGlobalGenerator::GetLanguageOutputExtension(
 {
   const std::string& lang = source.GetLanguage();
   if (!lang.empty()) {
-    auto const it = this->LanguageToOutputExtension.find(lang);
-    if (it != this->LanguageToOutputExtension.end()) {
-      return it->second;
-    }
-  } else {
-    // if no language is found then check to see if it is already an
-    // output extension for some language.  In that case it should be ignored
-    // and in this map, so it will not be compiled but will just be used.
-    std::string const& ext = source.GetExtension();
-    if (!ext.empty()) {
-      if (this->OutputExtensions.count(ext)) {
-        return ext;
-      }
+    return this->GetLanguageOutputExtension(lang);
+  }
+  // if no language is found then check to see if it is already an
+  // output extension for some language.  In that case it should be ignored
+  // and in this map, so it will not be compiled but will just be used.
+  std::string const& ext = source.GetExtension();
+  if (!ext.empty()) {
+    if (this->OutputExtensions.count(ext)) {
+      return ext;
     }
   }
   return "";
 }
 
+std::string cmGlobalGenerator::GetLanguageOutputExtension(
+  std::string const& lang) const
+{
+  auto const it = this->LanguageToOutputExtension.find(lang);
+  if (it != this->LanguageToOutputExtension.end()) {
+    return it->second;
+  }
+  return "";
+}
+
 std::string cmGlobalGenerator::GetLanguageFromExtension(const char* ext) const
 {
   // if there is an extension and it starts with . then move past the

Source/cmGlobalGenerator.h

@@ -349,6 +349,8 @@ public:
   int GetLinkerPreference(const std::string& lang) const;
   //! What is the object file extension for a given source file?
   std::string GetLanguageOutputExtension(cmSourceFile const&) const;
+  //! What is the object file extension for a given language?
+  std::string GetLanguageOutputExtension(std::string const& lang) const;
 
   //! What is the configurations directory variable called?
   virtual const char* GetCMakeCFGIntDir() const { return "."; }