Improved Visual Studio visualizations for template argument lists

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 *>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271944 91177308-0d34-0410-b5e6-96231b3b80d8