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("{ }", .{

src/codegen/llvm.zig

@@ -3722,15 +3722,11 @@ pub const Object = struct {
             => unreachable, // non-runtime values
             .extern_func => |extern_func| {
                 const fn_decl_index = extern_func.decl;
-                const fn_decl = mod.declPtr(fn_decl_index);
-                try mod.markDeclAlive(fn_decl);
                 const function_index = try o.resolveLlvmFunction(fn_decl_index);
                 return function_index.ptrConst(&o.builder).global.toConst();
             },
             .func => |func| {
                 const fn_decl_index = func.owner_decl;
-                const fn_decl = mod.declPtr(fn_decl_index);
-                try mod.markDeclAlive(fn_decl);
                 const function_index = try o.resolveLlvmFunction(fn_decl_index);
                 return function_index.ptrConst(&o.builder).global.toConst();
             },
@@ -4262,7 +4258,6 @@ pub const Object = struct {
     fn lowerParentPtrDecl(o: *Object, decl_index: InternPool.DeclIndex) Allocator.Error!Builder.Constant {
         const mod = o.module;
         const decl = mod.declPtr(decl_index);
-        try mod.markDeclAlive(decl);
         const ptr_ty = try mod.singleMutPtrType(decl.typeOf(mod));
         return o.lowerDeclRefValue(ptr_ty, decl_index);
     }
@@ -4455,8 +4450,6 @@ pub const Object = struct {
         if ((!is_fn_body and !decl_ty.hasRuntimeBits(mod)) or
             (is_fn_body and mod.typeToFunc(decl_ty).?.is_generic)) return o.lowerPtrToVoid(ty);
 
-        try mod.markDeclAlive(decl);
-
         const llvm_global = if (is_fn_body)
             (try o.resolveLlvmFunction(decl_index)).ptrConst(&o.builder).global
         else

src/codegen/spirv.zig

@@ -255,7 +255,6 @@ pub const Object = struct {
     pub fn resolveDecl(self: *Object, mod: *Module, decl_index: InternPool.DeclIndex) !SpvModule.Decl.Index {
         const decl = mod.declPtr(decl_index);
         assert(decl.has_tv); // TODO: Do we need to handle a situation where this is false?
-        try mod.markDeclAlive(decl);
 
         const entry = try self.decl_link.getOrPut(self.gpa, decl_index);
         if (!entry.found_existing) {