Implement syntactic merge checks for parameters. (#4149)

Note this isn't implementing checking through imports. The parse node
there is harder to access through the context, so would require
examining the entity in order to get the import declaration, to get at
the ImportIR. We also don't have a parse tree attached in that case, and
would need to add one to SemIR::File. But I believe we do want to add
that, so it's explicitly a TODO.

Note GetTokenText re-lexes literal values, so there's a bit of potential
overhead there. Not sure if we want a more efficient manner for
comparing in cases like this.

toolchain/parse/tree.cpp

@@ -32,6 +32,13 @@ auto Tree::postorder(NodeId n) const
       PostorderIterator(NodeId(end_index)));
 }
 
+auto Tree::postorder(NodeId begin, NodeId end) const
+    -> llvm::iterator_range<PostorderIterator> {
+  CARBON_CHECK(begin.is_valid() && end.is_valid());
+  return llvm::iterator_range<PostorderIterator>(
+      PostorderIterator(begin), PostorderIterator(NodeId(end.index + 1)));
+}
+
 auto Tree::children(NodeId n) const -> llvm::iterator_range<SiblingIterator> {
   CARBON_CHECK(n.is_valid());
   int end_index = n.index - node_impls_[n.index].subtree_size;