[clang][reflection] Mangle deduction-guide reflections instead of hitting unreachable

[Cfretz244]

Fixes #298.

Stacked on #287 — the first commit here is #287's (this change extends the same mangleReflection hash block); review the last commit only.

Problem

members_of over a namespace enumerates deduction guides like any other member, and lifting that member list into define_static_array mangles each element reflection as a template argument of the backing array specialization. mangleReflection's Template case encodes a reflected template via mangleTemplateName → mangleUnqualifiedName, which has no case for CXXDeductionGuideName: llvm_unreachable("Can't mangle a deduction guide name!", ItaniumMangle.cpp:1774) — a sound invariant for normal symbol mangling (guides are never odr-used) that reflection now makes reachable from ordinary user code. Field shape: TartanLlama/expected's namespace-scope template <class E> unexpected(E) -> unexpected<E>; — a reflection-driven binding generator walking the tl namespace ICE'd at codegen while binding ANY tl class (see #298 for the reproducer + build matrix).

Fix

clang/lib/AST/ItaniumMangle.cpp, local to mangleReflection's ReflectionKind::Template case (the mangleUnqualifiedName unreachable stays): a guide reflection encodes as "dg" + the deduced template's name + the same '$'-bracketed ODR-hash discriminator used for overloaded function templates (#287). Two subtleties force more than crash avoidance:

• every guide for one template shares the single CXXDeductionGuideName, and Sema's implicit guides (per-constructor + the copy guide) are enumerated alongside explicit ones once CTAD has been used in the TU;
• an implicit per-constructor guide can be structurally identical to a same-signature explicit guide (and the copy guide to a per-constructor guide for X(X<E>)), so the hash also folds in isImplicit() and the deduction-candidate kind.

Deterministic and cross-TU-stable (ODR hash), preserving legitimate linkonce_odr merging; distinct from a reflection of the deduced class template itself (no dg tag).

Test

libcxx/test/std/experimental/reflection/deduction-guide-reflection-mangling.pass.cpp (new): the define_static_array(members_of(...)) lift shape (the ICE), with four guides in the enumeration — two explicit (one structurally identical to the implicit per-constructor guide) + two implicit — pinned as &probe<m> NTTPs and asserted pairwise distinct at runtime, plus distinct from the deduced class template's own reflection. A fold is invisible to static_assert (the AST is always correct), so the observations are runtime.

Validation

• Base: [clang][reflection] Discriminate same-named function-template reflections in NTTP mangling #287's branch tip (caac14845397, itself on 837da39eb88c); builds standalone.
• Without the fix: the Itanium mangler ICE ("Can't mangle a deduction guide name!") mangling a reflection of a deduction guide as a template argument / define_static_array element #298 reproducer ICEs at -c (clean at -fsyntax-only and without the guide); the new test ICEs.
• With the fix: reproducer matrix fully clean; new test passes; [clang][reflection] Discriminate same-named function-template reflections in NTTP mangling #287's own regression test (substitute-nested-dependent.pass.cpp) still passes on the stack.
• clang/test/Reflection: 16/16 with the fix on this machine — identical to base.
• Downstream soak (reflection-driven nanobind binding generator): 53-test binder suite green; TartanLlama/expected v1.3.1 corpus run — formerly ICE'ing at codegen on every tl class — passes its full differential suite.

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