Encapsulate template arguments lists in a new class,

Encapsulate template arguments lists in a new class,
TemplateArgumentList. This avoids the need to pass around
pointer/length pairs of template arguments lists, and will eventually
make it easier to introduce member templates and variadic templates.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@71517 91177308-0d34-0410-b5e6-96231b3b80d8