Log Message:
bison: update to 3.7.5.

* Noteworthy changes in release 3.7.5 (2021-01-24) [stable]

** Bug fixes

*** Counterexample Generation

  In some cases counterexample generation could crash.  This is fixed.

*** Fix Table Generation

  In some very rare conditions, when there are many useless tokens, it was
  possible to generate incorrect parsers.

*** GLR parsers now support %merge together with api.value.type=union.

*** C++ parsers use noexcept in more places.

*** Generated parsers avoid some warnings about signedness issues.

*** C-language parsers now avoid warnings from pedantic clang.

*** C-language parsers now work around quirks of HP-UX 11.23 (2003).