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

[Richard Smith via cfe-commits]

zygoloid wrote:

Conversion function templates might be a special case here because of how conversion function template specializations are found by deduction as part of class member name lookup. Maybe we're doing something different in that deduction in particular that means we're not properly lazily loading them? 

If this issue is indeed specific to conversion function templates, perhaps a path forward would be to disable some part of the lazy loading for only those templates to unblock this patch while the reason for the problem is being investigated?

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