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_variable.cpp

@@ -95,6 +95,9 @@ auto HandleVariableDecl(Context& context, Parse::VariableDeclId parse_node)
   }
 
   context.decl_state_stack().Pop(DeclState::Var);
+  if (context.scope_stack().PeekIndex() == ScopeIndex::Package) {
+    context.inst_block_stack().PopGlobalInit();
+  }
 
   return true;
 }