[clang][reflection] Handle entity-proxy reflections in member metafunctions and NTTP mangling

[Cfretz244]

Fixes #290.

Problem

With -fentity-proxy-reflection, members_of enumerates using-shadow declarations as ReflectionKind::EntityProxy, so proxy reflections flow directly into ordinary consteval member queries and, as std::meta::info NTTPs, into the Itanium mangler. Several consumers still treated that kind as llvm_unreachable("proxies should already have been unwrapped"), turning valid code into ICEs (see #290 for self-contained reproducers and crash signatures).

Fix

• clang/lib/AST/ExprConstantMeta.cpp — six metafunctions crashed when asked of a proxy: is_constructor, is_destructor, is_special_member_function, is_static_member, is_enumerable_type, has_complete_definition. Each now answers false, consistent with how the surrounding kind predicates already classify proxies (a shadow declaration is never itself one of these; the underlying entity's properties are available through underlying_entity_of). The remaining unreachable proxy arms were probed NOT reachable from user code (identifier_of/has_identifier/source_location_of pre-unwrap via MaybeUnproxy; substitute's dispatcher unwraps template arguments before TArgFromReflection; reflect_invoke through a proxy fails gracefully as not-a-constant-expression) and are left as-is.
• clang/lib/AST/ItaniumMangle.cpp — mangleReflection's EntityProxy case encoded the proxy by mangling the shadow declaration's NAME (mangleNameWithAbiTags), with two defects: an operator-named shadow (e.g. using OB::operator*;, the shape absl::StatusOr<T> hits via using StatusOr::OperatorBase::operator*;) crashed outright (mangleUnqualifiedName casts the decl to FunctionDecl to disambiguate the operator name — cast<> assertion), and one using-declarator over an overload set introduced several same-named shadows whose proxies all mangled identically ("definition with same mangled name" at best, a silent linkonce_odr fold at worst). The mangler now encodes the proxy's TARGET declaration kind-aware (ctors/dtors via GlobalDecl, functions/variables/fields via mangle(), enum constants as literals, types via mangleCanonicalTypeName, else the source name), keeping the a tag so a proxy-of-X reflection stays a distinct template argument from a declaration-of-X reflection.

Test

• libcxx/test/std/experimental/reflection/entity-proxy-member-queries.pass.cpp (new): a consteval sweep of all six metafunctions over proxies of every member kind — member function, static member function, static data member, field, type alias, nested class (each call ICEd before the fix) — plus runtime distinctness assertions for proxy NTTPs: an operator shadow and a four-overload value() shadow set from a class template specialization, asserting every proxy instantiation is distinct from its siblings and from its underlying declaration's instantiation (static_assert cannot catch a mangling fold).

Validation

• Base: 837da39eb88c (current p2996 tip; applies cleanly).
• Without the fix: each of the six metafunction calls independently hits the UNREACHABLE (ExprConstantMeta.cpp:5240/5391/5435/4563/4830/4789); the operator-shadow NTTP asserts in mangleUnqualifiedName; the overload-set NTTP errors with definition with same mangled name '_Z5probeIMaN1SIiE5valueE$EEiv'; the new test ICEs.
• With the fix: reproducers and the new test compile and run clean.
• clang/test/Reflection: 15/16 both at base and with the fix — identical results; the one failure (splice-exprs.cpp) is pre-existing at base and unrelated.
• Downstream soak (reflection-driven nanobind binding generator): 50-test binder suite with all modules recompiled, plus real-library corpus runs including absl::StatusOr (the field trigger: its using StatusOr::OperatorBase::value/operator*/operator->; re-exports) and nlohmann/json — all green.

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