Support extended C++20 template-param qualifier forms

[kristjanvalur]

Summary

• support additional C++20 template-parameter qualifier mangling forms

Motivation and Context

This is one part of a larger C++20 Itanium compatibility effort derived from real-world symbol corpus failures.

These C++20-oriented changes are intentionally split into three PRs for reviewability:

• PR Render requires clauses with level-aware template params #314: level-aware template parameter resolution + requires-clause rendering
• PR Support extended C++20 template-param qualifier forms #315: extended template-parameter qualifier forms
• PR Normalize local-source template handling in nested names #316: local-source template handling and nested-name normalization

This PR isolates qualifier-form parsing support so reviewers can evaluate syntax-surface changes independently.

What "Template-Parameter Qualifier Mangling" Means

In this PR, "template-parameter qualifier mangling" refers to Itanium mangling markers that qualify or annotate a template parameter declaration while encoding template arguments (for example forms using Tk, Tt, Tp, Ty).

These encodings are related to C++20 source-level constructs (such as constrained template parameters, abbreviated templates, and placeholder auto forms), but they are not always a 1:1 printable syntax token from source text. In practice:

• some forms correspond fairly directly to C++ syntax concepts (for example constrained/placeholder parameter declarations), and
• some appear as ABI/compiler-lowering detail used to preserve semantic context during mangling.

So the short answer is: they are used when mangling certain C++20 constructs, and may represent either direct source-level intent or normalized compiler-internal encoding detail, depending on context.

Scope

• parser support for extended C++20 template-parameter qualifier encodings