SL-19572 Fix some obscure cases of residents bypasing mute list

indra/llmessage/llcachename.cpp

@@ -552,7 +552,9 @@ std::string LLCacheName::buildUsername(const std::string& full_name)
 
 	// if the input wasn't a correctly formatted legacy name, just return it  
 	// cleaned up from a potential terminal "Resident"
-	return cleanFullName(full_name);
+    std::string clean_name = cleanFullName(full_name);
+    LLStringUtil::toLower(clean_name);
+	return clean_name;
 }
 
 //static 

indra/newview/llmutelist.h

@@ -99,6 +99,7 @@ public:
 	BOOL isMuted(const LLUUID& id, const std::string& name = LLStringUtil::null, U32 flags = 0) const;
 
 	// Workaround for username-based mute search, a lot of string conversions so use cautiously
+    // Expects lower case username
 	BOOL isMuted(const std::string& username, U32 flags = 0) const;
 
 	// Alternate (convenience) form for places we don't need to pass the name, but do need flags