Fixup build warnings

Change-Id: I7a61bd4363b01bf186e235bf76e97bd513d2fd12
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/26081
Reviewed-by: Ryan Harrison <rharrison@google.com>

src/ast/block_statement.h

@@ -46,7 +46,8 @@ class BlockStatement : public Statement {
   /// @param index the index to insert at
   /// @param stmt the statement to insert
   void insert(size_t index, std::unique_ptr<ast::Statement> stmt) {
-    statements_.insert(statements_.begin() + index, std::move(stmt));
+    statements_.insert(statements_.begin() + static_cast<long>(index),
+                       std::move(stmt));
   }
 
   /// @returns true if the block is empty

src/reader/wgsl/parser_impl.h

@@ -143,6 +143,7 @@ class ParserImpl {
   /// @returns the struct decoration or StructDecoraton::kNone
   ast::StructDecoration struct_decoration_decl();
   /// Parses a `struct_decoration` grammar element
+  /// @param t the current token
   /// @returns the struct decoration or StructDecoraton::kNone if none matched
   ast::StructDecoration struct_decoration(Token t);
   /// Parses a `struct_body_decl` grammar element