Improve the representation of template type parameters. We now

Improve the representation of template type parameters. We now
canonicalize by template parameter depth, index, and name, and the
unnamed version of a template parameter serves as the canonical.

TemplateTypeParmDecl no longer needs to inherit from
TemplateParmPosition, since depth and index information is present
within the type.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63899 91177308-0d34-0410-b5e6-96231b3b80d8