nightly

Auto merge of #14580 - lowr:patch/parse-more-unstable-pat, r=Veykril

Parse more exclusive range patterns and inline const patterns

Closes #13955

This PR
- implements exclusive range pattern without start bound (tracking issue: rust-lang/rust#37854)
  - additionally moves rest pattern handling into the same place since they only differ in whether another pattern follows; this actually solves some FIXMEs
- updates `PATTERN_FIRST` token set to include `const` token so we can parse inline const pattern in nested patterns