Convert many functions to use StringRefs.

Where possible, remove the const char* version.  To keep the
risk and impact here minimal, I've only done the simplest
functions.

In the process, I found a few opportunities for adding some
unit tests, so I added those as well.

Tested on Windows, Linux, and OSX.

llvm-svn: 281799
diff --git a/lldb/source/Interpreter/OptionValueLanguage.cpp b/lldb/source/Interpreter/OptionValueLanguage.cpp
index f5505f7..a59419f 100644
--- a/lldb/source/Interpreter/OptionValueLanguage.cpp
+++ b/lldb/source/Interpreter/OptionValueLanguage.cpp
@@ -49,7 +49,7 @@
                                                 languages_for_expressions);
 
     LanguageType new_type =
-        Language::GetLanguageTypeFromString(lang_name.GetCString());
+        Language::GetLanguageTypeFromString(lang_name.GetStringRef());
     if (new_type && languages_for_types.count(new_type)) {
       m_value_was_set = true;
       m_current_value = new_type;