[lldb] Add lookup failures to TypeSystemSwiftTypeRef's clang type cache

By profiling LLDB running frame variable in a large application with
many many .o files, I noticed that one of the biggest bottlenecks of the
operation was looking (and failing) to find a certain potential clang
type over and over again.

Author: augusto2112

lldb/source/Plugins/TypeSystem/Swift/TypeSystemSwiftTypeRef.cpp

@@ -451,6 +451,10 @@ TypeSP TypeSystemSwiftTypeRefForExpressions::LookupClangType(
   if (TargetSP target_sp = GetTargetWP().lock())
     target_sp->GetImages().ForEach(lookup);
 
+  /// Cache the negative result. This is safe to do because ModulesDidLoad will
+  /// clear the cache.
+  if (!result)
+    m_clang_type_cache.Insert(name.AsCString(), result);
   return result;
 }