Destroy temporaries at the end of expression statements. (#7513)

Instead of tracking the cleanup scope depth on entry to each scope,
track an "ambient" cleanup scope depth that's *after* the destructors of
local variables in that scope. This gets increased to include the
destructors of local variables when we create a name-binding
declaration. Then, when we reach a point where temporaries should be
destroyed, run cleanups that are after the ambient cleanup scope depth
on the stack. This happens:

* At the `;` of a statement expression.
* At the `)` of an `if` or `while` statement.
* After performing the implied `HasValue()` call in a `for` statement.

Per informal agreement with leads, this means we lifetime-extend all
temporaries created in the initializer of a name-binding declaration to
the full scope of that declaration, but that temporaries created in an
expression statement are destroyed at the `;`.

toolchain/check/scope_stack.cpp

@@ -210,8 +210,9 @@ auto ScopeStack::MergeTopScopeIntoGrandparentAndPop() -> void {
       std::rotate(destroy_id_stack_.begin() + parent.cleanup_scope_depth.index,
                   destroy_id_stack_.begin() + current.cleanup_scope_depth.index,
                   destroy_id_stack_.end());
-  parent.cleanup_scope_depth.index = new_mid - destroy_id_stack_.begin();
-  current.cleanup_scope_depth.index = destroy_id_stack_.size();
+  parent.cleanup_scope_depth =
+      CleanupScopeDepth(new_mid - destroy_id_stack_.begin());
+  current.cleanup_scope_depth = CleanupScopeDepth(destroy_id_stack_.size());
 
   Pop();
 }