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/control_flow.cpp

@@ -161,7 +161,13 @@ static auto AddCleanups(Context& context, ScopeStack::CleanupScopeDepth depth)
   }
 }
 
-auto AddAndDiscardCleanups(Context& context) -> void {
+auto AddAndDiscardTemporaryCleanups(Context& context) -> void {
+  auto depth = context.scope_stack().ambient_cleanup_scope_depth();
+  AddCleanups(context, depth);
+  context.scope_stack().DiscardCleanupsSince(depth);
+}
+
+auto AddAndDiscardScopeCleanups(Context& context) -> void {
   auto depth = context.scope_stack().enclosing_cleanup_scope_depth();
   AddCleanups(context, depth);
   context.scope_stack().DiscardCleanupsSince(depth);