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/Property.cpp b/lldb/source/Interpreter/Property.cpp
index 0030d37..ba859a5 100644
--- a/lldb/source/Interpreter/Property.cpp
+++ b/lldb/source/Interpreter/Property.cpp
@@ -139,7 +139,8 @@
     {
       LanguageType new_lang = eLanguageTypeUnknown;
       if (definition.default_cstr_value)
-        Language::GetLanguageTypeFromString(definition.default_cstr_value);
+        Language::GetLanguageTypeFromString(
+            llvm::StringRef(definition.default_cstr_value));
       else
         new_lang = (LanguageType)definition.default_uint_value;
       m_value_sp.reset(new OptionValueLanguage(new_lang));