Remove last uses of gensyms
Underscore bindings now use unique `SyntaxContext`s to avoid collisions. This was the last use of gensyms in the compiler, so this PR also removes them.
closes#49300
cc #60869
r? @petrochenkov
It means an allocation is required to create an empty `TokenStream`, but
all other operations are simpler and marginally faster due to not having
to check for `None`. Overall it simplifies the code for a negligible
performance effect.
The commit also removes `TokenStream::empty` by implementing `Default`,
which is now possible.
They are only used by rustc_lexer, and are not needed elsewhere.
So we move the relevant definitions into rustc_lexer (while the actual
unicode data comes from the unicode-xid crate) and make the rest of
the compiler use it.
Replace them with equivalents of `Span::{def_site,call_site}` from proc macro API.
The new API is much less error prone and doesn't rely on macros having default transparency.
