[lldb][NFC] Remove several inefficient ConstString -> const char * -> StringRef conversions

StringRef will call strlen on the C string which is inefficient (as ConstString already
knows the string lenght and so does StringRef). This patch replaces all those calls
with GetStringRef() which doesn't recompute the length.
diff --git a/lldb/source/Utility/StructuredData.cpp b/lldb/source/Utility/StructuredData.cpp
index c003412..df1b356 100644
--- a/lldb/source/Utility/StructuredData.cpp
+++ b/lldb/source/Utility/StructuredData.cpp
@@ -156,7 +156,7 @@
 void StructuredData::Dictionary::Serialize(json::OStream &s) const {
   s.objectBegin();
   for (const auto &pair : m_dict) {
-    s.attributeBegin(pair.first.AsCString());
+    s.attributeBegin(pair.first.GetStringRef());
     pair.second->Serialize(s);
     s.attributeEnd();
   }