libsyntax: Name can be made NonZero

Spawned from https://github.com/rust-lang/rust/pull/28642

`Name` is an index into the string interner's internal array. The first element of this array can be reserved and `Name` will always contain non-zero index. It will make `Option<Name>` and `Option<Ident>` commonly encountered in AST/HIR more compact.
Requires `const unsafe` functions.
