All units share one link namespace (per-unit isolation is PLAN-C C3.2,
deferred), so a symbol defined by two units previously died inside the
JIT dylib link or the AOT link with raw linker spew. The clang shim
gains sx_clang_object_exported_symbols (llvm::object scan: defined +
global, format-specific excluded) and compileCToObjects cross-checks
every unit object — collisions name both source files. Scan failures
are non-fatal; the linker remains the backstop. Covers JIT and native
AOT; the emcc path still relies on wasm-ld's own error.
