Improved Visual Studio visualizations for template argument lists

Improved the visualizer for TemplateArgumentList to show type arguments in the DisplayString.
E.g., <double, long>. Added a visualizer for MultiLevelTemplateArgumentList.
I decided to display them by how they would appear in a template with the
(non-existent) template-id's omitted, so the DisplayString naturally presents
as something like <double, long>::<char *>.

llvm-svn: 271944