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/inst_block_stack.cpp

@@ -21,6 +21,11 @@ auto InstBlockStack::Push(SemIR::InstBlockId id) -> void {
   ++size_;
 }
 
+auto InstBlockStack::PushGlobalInit() -> void {
+  Push(SemIR::InstBlockId::GlobalInit);
+  stack_[size_ - 1].content = std::move(init_block_);
+}
+
 auto InstBlockStack::PeekOrAdd(int depth) -> SemIR::InstBlockId {
   CARBON_CHECK(size() > depth) << "no such block";
   int index = size() - depth - 1;
@@ -52,6 +57,14 @@ auto InstBlockStack::Pop() -> SemIR::InstBlockId {
   return back.id;
 }
 
+auto InstBlockStack::PopGlobalInit() -> void {
+  CARBON_CHECK(stack_[size_ - 1].id == SemIR::InstBlockId::GlobalInit)
+      << "Trying to pop Inits block from " << name_
+      << " but a different block is present!";
+  init_block_ = std::move(stack_[size_ - 1].content);
+  PopAndDiscard();
+}
+
 auto InstBlockStack::PopAndDiscard() -> void {
   CARBON_CHECK(!empty()) << "no current block";
   --size_;