Re-commit [clang] Add support for the "abstract" contextual keyword of MSVC

https://docs.microsoft.com/en-us/cpp/extensions/abstract-cpp-component-extensions?view=msvc-160
Note: like the already supported "sealed" keyword, the "abstract"
keyword is supported by MSVC by default.

This re-commits 818338ad with added
initialization of Parser::Ident_abstract.

Differential revision: https://reviews.llvm.org/D102517