Fix various proc-macro bugs

[edwin0cheng]

This PRs does the following things:

1. Fixed thread '<unnamed>' panicked at 'called Option::unwrap() on a None value', crates/ra_mbe/src/subtree_source.rs:155:16 #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 thread '<unnamed>' panicked at 'MacroDef without ast_id', crates/ra_hir/src/has_source.rs:109:22 #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.

[edwin0cheng]

I found that serde defined a try! macro in the expanded code such that it is syntax error (#2241) in RA currently.

[matklad]

bors r+

Would be good to add a test for lifetime splitting though!

[bors]

Build succeeded:

• TypeScript
• Rust (macos-latest)
• Rust (windows-latest)
• Rust (ubuntu-latest)

[edwin0cheng]

lifetime splitting test added in #4032