Rollup merge of #143558 - joshtriplett:mbe-refactors, r=SparrowLii

mbe: Refactors and function extractions in `compile_declarative_macro`

These refactors help pave the way for parsing attribute rules.

Best reviewed commit-by-commit.

- **mbe: Simplify compile_declarative_macro by factoring out some variables**
- **mbe: Factor out a helper to check an LHS**
- **mbe: Factor out a helper to check for unexpected EOF in definition**
- **mbe: Clarify comments about error handling in `compile_declarative_macro`**

Author: jieyouxu