Further refactoring of the tree transformation for template argument

Further refactoring of the tree transformation for template argument
lists, so that all such transformations go through a single,
iterator-based transformation function. This is the only place where
we need to implement the logic for transforming pack expansions whose
pattern is a template argument.

Unfortunately, the new cases this refactoring brings into the fold
can't be meaningfully tested yet. We need template argument deduction
to work well enough for variadic function templates first.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122289 91177308-0d34-0410-b5e6-96231b3b80d8