7827: Fix proc macro TokenStream::from_str token ids r=vlad20012 a=vlad20012
To be honest, I don't know what it changes from a user perspective.
Internally, this fixes spans (token ids) of a `TokenStream` parsed from a string:
```rust
#[proc_macro_derive(FooDerive)]
pub fn foo_derive(item: TokenStream) -> TokenStream {
"fn foo() {}".parse().unwrap()
}
```
Previously, `TokenStream` was constructed from tokens with incremental ids (that conflicted with call-site tokens). Now they are `-1`.
Co-authored-by: vlad20012 <beskvlad@gmail.com>
477c1ac105