Fix a crash when checking a nested tuple-pattern (#7716)

Adds a virtual node to `DefaultValuePattern` to end the
`ExprRegionForPattern` before checking the expression
for the default value.

When checking the default value expression, the context
was still configured to interpret expressions as patterns,
which caused some corruption of state with tuple-pattern
subpatterns.

Corrects an assertion failure I found while working on
feedback from #7665.

toolchain/parse/handle_pattern_list.cpp

@@ -175,6 +175,10 @@ auto HandlePatternListElementCheckForDefaultValue(Context& context) -> void {
     return;
   }
 
+  // Add the first virtual node surrounding the value expr node, to facilitate
+  // handling in check.
+  context.AddLeafNode(NodeKind::DefaultValueExprStart, *equals_token);
+
   state.token = *equals_token;
   state.kind = StateKind::PatternListElementFinishDefaultValue;
   context.PushState(state);