Add GlobalInit to blocks to emit initialization instructions for globals (#3687)

`GlobalInit` block is now static block within a `SemIR` which will be
used to emit initialization instructions for variables in the `Package`
scope.

inst_block_stack now has additional methods to handle `GlobalInit` block
separately, this block can be popped without being finalized allowing to
accumulate between all instances of variables.

At the end of the `check` phase, if this block is not empty , the
function `__global_init` will be added with this block being inserted
into it.

This block is pushed to `inst_block_scope` at the end `BindName`,
allowing instruction to be emitted into it, then popped at the semicolon
(VariableDecl).

This significantly changes the `SemIR` output, that's why this commit
updates a lot of the test cases.

toolchain/check/handle_binding_pattern.cpp

@@ -116,6 +116,9 @@ auto HandleAnyBindingPattern(Context& context, Parse::NodeId parse_node,
       if (context_parse_node_kind == Parse::NodeKind::ReturnedModifier) {
         RegisterReturnedVar(context, bind_id);
       }
+      if (context.scope_stack().PeekIndex() == ScopeIndex::Package) {
+        context.inst_block_stack().PushGlobalInit();
+      }
       break;
     }