As agreed on https://github.com/itanium-cxx-abi/cxx-abi/issues/109 these
placeholders should be mangled as a `template-prefix` production.
```
<template-prefix> ::= <template unqualified-name> # global template
::= <prefix> <template unqualified-name> # nested template
::= <template-param> # template template parameter
::= <substitution>
```
Previous to this patch, the template template parameter case was not
handled, and template template parameters were incorrectly being handled
as unqualified-names.
Before #95202, DeducedTemplateType was not canonicalized correctly, so
that template template parameter declarations were retained
uncanonicalized.
After #95202, they are correctly canonicalized, but this now leads to
these TTPs being anonymous entities, where the mangling implementation
correctly doesn't expect an anonymous declaration of this kind, leading
to a crash.
Fixes#106182.
