[serialization] PR34728: Don't assume that only a suffix of template

[serialization] PR34728: Don't assume that only a suffix of template
parameters can have default arguments.

At least for function templates and class template partial
specializations, it's possible for a template parameter with a default
argument to be followed by a non-pack template parameter with no default
argument, and this case was not properly handled here.

Testcase by Steve O'Brien!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@338438 91177308-0d34-0410-b5e6-96231b3b80d8