Add Rust support to Mangled

This adds Rust support to Mangled.  I am not completely certain that
this is needed (or alternatively that it does enough, maybe
Mangled::GuessLanguage needs a Rust case).  This should be checked
before attempting to upstream.

Author: tromey

lldb/source/Core/Mangled.cpp

@@ -257,6 +257,11 @@ ConstString Mangled::GetDemangledName() const {
         break;
       case eManglingSchemeItanium: {
         demangled_name = GetItaniumDemangledStr(mangled_name);
+
+        if (language == lldb::eLanguageTypeRust) {
+          remove_rust_hash(demangled_name);
+        }
+
         break;
       }
       case eManglingSchemeRustV0: