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/check.cpp

@@ -262,6 +262,7 @@ static auto CheckParseTree(
   sem_ir.set_top_inst_block_id(context.inst_block_stack().Pop());
   context.scope_stack().Pop();
   context.FinalizeExports();
+  context.FinalizeGlobalInit();
 
   context.VerifyOnFinish();