[lldb] Fix another instance where we pass a nullptr as TypeSourceInfo to...

[lldb] Fix another instance where we pass a nullptr as TypeSourceInfo to NonTypeTemplateParmDecl::Create

Summary:
Follow up to an issue pointed out in the review of D73808. We shouldn't just pass in a nullptr TypeSourceInfo
in case Clang decided to access it.

Reviewers: shafik, vsk

Reviewed By: shafik, vsk

Subscribers: kristof.beyls, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D73946