4029: Fix various proc-macro bugs r=matklad a=edwin0cheng
This PRs does the following things:
1. Fixed#4001 by splitting `LIFETIME` lexer token to two mbe tokens. It is because rustc token stream expects `LIFETIME` as a combination of punct and ident, but RA `tt:TokenTree` treats it as a single `Ident` previously.
2. Fixed#4003, by skipping `proc-macro` for completion. It is because currently we don't have `AstNode` for `proc-macro`. We would need to redesign how to implement `HasSource` for `proc-macro`.
3. Fixed a bug how empty `TokenStream` merging in `proc-macro-srv` such that no L_DOLLAR and R_DOLLAR will be emitted accidentally.
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
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