Create an array stack type for a shared use-case (#4100)

Based on discussion around the region handling in generic_region_stack,
create a generic structure for the stack-of-vectors support. I also want
to add this to InstBlockStack, but that's a little more complex due to
GlobalInit, so cutting a PR here to check with review.

My work here is how I noticed #4099; I want to be sure that I'm correct
about the issue, but it's the difference between being able to use
PeekArray or not.

Note in scope_stack.h, I believe we could remove next_compile_time_index
and make it just based on elements_size(). However, I want to verify
with you before I make further changes there.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/check/generic_region_stack.cpp

@@ -6,27 +6,16 @@
 
 namespace Carbon::Check {
 
-auto GenericRegionStack::Push() -> void {
-  regions_.push_back(
-      {.first_dependent_inst = static_cast<int32_t>(dependent_insts_.size())});
-}
+auto GenericRegionStack::Push() -> void { dependent_insts_stack_.PushArray(); }
 
-auto GenericRegionStack::Pop() -> void {
-  auto region = regions_.pop_back_val();
-  dependent_insts_.truncate(region.first_dependent_inst);
-}
+auto GenericRegionStack::Pop() -> void { dependent_insts_stack_.PopArray(); }
 
 auto GenericRegionStack::AddDependentInst(DependentInst inst) -> void {
-  CARBON_CHECK(!regions_.empty())
-      << "Formed a dependent instruction while not in a generic region.";
-  CARBON_CHECK(inst.kind != DependencyKind::None);
-  dependent_insts_.push_back(inst);
+  dependent_insts_stack_.AppendToTop(inst);
 }
 
 auto GenericRegionStack::PeekDependentInsts() -> llvm::ArrayRef<DependentInst> {
-  CARBON_CHECK(!regions_.empty());
-  return llvm::ArrayRef(dependent_insts_)
-      .slice(regions_.back().first_dependent_inst);
+  return dependent_insts_stack_.PeekArray();
 }
 
 }  // namespace Carbon::Check