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/handle_if_statement.cpp

@@ -20,6 +20,9 @@ auto HandleParseNode(Context& context, Parse::IfConditionId node_id) -> bool {
   auto cond_value_id = context.node_stack().PopExpr();
   cond_value_id = ConvertToBoolValue(context, node_id, cond_value_id);
 
+  // Destroy any temporaries created in the condition.
+  AddAndDiscardTemporaryCleanups(context);
+
   // Create the then block and the else block, and branch to the right one. If
   // there is no `else`, the then block will terminate with a branch to the
   // else block, which will be reused as the resumption block.