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_codeblock.cpp

@@ -17,7 +17,7 @@ auto HandleParseNode(Context& context, Parse::CodeBlockStartId node_id)
 }
 
 auto HandleParseNode(Context& context, Parse::CodeBlockId /*node_id*/) -> bool {
-  AddAndDiscardCleanups(context);
+  AddAndDiscardScopeCleanups(context);
   context.scope_stack().Pop(/*check_unused=*/true);
   context.node_stack()
       .PopAndDiscardSoloNodeId<Parse::NodeKind::CodeBlockStart>();