ast::Literal constructor is explicit

Fixes a lint issue

Change-Id: I98225fe5bf966c6fbf9b3b0c88e3d4dcc6a91cca
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/18041
Reviewed-by: dan sinclair <dsinclair@google.com>

src/ast/literal.h

@@ -61,7 +61,7 @@ class Literal {
 
  protected:
   /// Constructor
-  Literal(ast::type::Type* type);
+  explicit Literal(ast::type::Type* type);
 
  private:
   ast::type::Type* type_ = nullptr;