fix(cpp): an alias of a type written from the global scope names that global type

[colbymchenry]

What

This is the hpb/arena.h follow-up from #2567. protocolbuffers/protobuf's hpb declares class Arena { … backend::Arena arena_; }, where:

• namespace backend = hpb::internal::backend::upb; (in hpb/multibackend.h);
• namespace hpb::internal::backend::upb { using Arena = ::upb::Arena; } (in hpb/backend/types.h).

So arena_.Fuse(other.arena_) in hpb::Arena::Fuse should reach upb::Arena::Fuse. On main, hpb::Arena::Fuse, IsFused and RefArena each linked to themselves, at 0.9.

Cause: cppTypeName drops a leading ::. Following the alias looked its target up as upb::Arena from the alias's own scope, hpb::internal::backend::upb, where the nearest upb is that namespace itself. The alias named itself, the lookup ended on no class, and the call fell back to the type's last name. That fallback prefers the call site's file, which here is the calling method.

Fix: TypeName records a leading :: (global), and a global name is looked up from the global scope alone in each place that resolves one:

• following an alias's target (follow, and the scope expand looks the target up from and hands on);
• a class's base clause (baseClassesOf);
• cppAliasedTypeWritten, which fix(cpp): a call on an auto variable or a chained call reaches the class C++ finds, not a namesake #2532's auto locals and chains walk alias by alias.

cppClassNamed needed nothing, since its callers already pass only the global scope for :: names. A receiver's declared type never carries a leading ::, because the declarator pattern starts at a letter, so nothing changes there.

Validation

Tests: __tests__/cpp-global-alias-target.test.ts, 4 tests, all failing on main:

• the hpb chain;
• an alias to ::upb::Arena beside a namespace-local upb::Arena (main: hpb::upb::Arena::Fuse);
• an auto variable typed through such an alias (main: the caller's file's hpb::Arena::Fuse);
• a base class written ::lib::Options beside a nested app::lib::Options (main: app::Inner::Run).

All 42 C/C++ test files pass (396 tests). Full suite: 7,123 passed, 0 failed (82 skipped).

Mutations: each change, switched off, fails its own test.

Mutation	Tests that fail
global never set	all 4
follow lookup scopes	hpb chain, auto
expand's follow global	hpb chain
expand's lookup scope	namespace-local namesake
base clause	::lib::Options
cppAliasedTypeWritten	auto

Site-keyed A/B (main 00d7430 vs this branch, kit %TEMP%\cgstd, diff-sites.mjs): leveldb, fmt, redis and rocksdb are byte-identical. protobuf has exactly 3 retargets, all right:

• hpb/arena.h:38 arena_.Fuse: hpb::Arena::Fuse (itself) → upb::Arena::Fuse
• hpb/arena.h:39 arena_.IsFused: hpb::Arena::IsFused (itself) → upb::Arena::IsFused
• hpb/arena.h:50 arena_.RefArena: hpb::Arena::RefArena (itself) → upb::Arena::RefArena

CHANGELOG: #2399's [Unreleased] alias bullet was extended rather than given a new one, since it isn't released yet.

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