Zcu: eliminate Decl.alive field

Legacy anon decls now have three uses:
* Type owner decls
* Function owner decls
* `@export` and `@extern`

Therefore, there are no longer any cases where we wish to explicitly
omit legacy anon decls from the binary. This means we can remove the
concept of an "alive" vs "dead" `Decl`, which also allows us to remove
the separate `anon_work_queue` in `Compilation`.

src/codegen/c.zig

@@ -2010,7 +2010,6 @@ pub const DeclGen = struct {
     fn renderDeclName(dg: *DeclGen, writer: anytype, decl_index: InternPool.DeclIndex, export_index: u32) !void {
         const mod = dg.module;
         const decl = mod.declPtr(decl_index);
-        try mod.markDeclAlive(decl);
 
         if (mod.decl_exports.get(decl_index)) |exports| {
             try writer.print("{ }", .{