[clang] [Serialization] Fix lazy template loading (PR #133057)

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> When I have time, I will need to see that I turn the reduced example into actual valid code to understand what is going on...

I spent some time turning it into valid code, but then noticed that the `error: use of overloaded operator '=' is ambiguous` is gone. Going back, the piece that triggers the error is the token `bns` in `TR7.cc`, which leads to `error: unknown type name 'bns'` and then messes up parsing of `namespace sr` in `UL8.h`. I believe the reproducer is not useful in the current form, you will have to instruct the reduce process to keep somewhat valid code...

https://github.com/llvm/llvm-project/pull/133057