Render requires clauses with level-aware template params

[kristjanvalur]

Summary

• add level-aware template parameter resolution behavior
• render requires-clause expressions in template argument contexts

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 focuses only on resolution/renders required to correctly represent C++20-style template argument contexts.

C++20 Example

template <typename T>
requires (sizeof(T) > 4)
void f(T);

A corresponding mangled form can encode the constraint expression as a Q <expression> segment; this PR ensures those requires-clause expressions are rendered with level-aware template parameter resolution.

Scope

• level-aware template parameter lookup/resolution improvements
• requires-clause expression rendering in relevant template-arg flows