[ASTMatchers] Add `cxxBaseSpecifier` matcher (non-top-level)

Required for capturing base specifier in matchers:
  `cxxRecordDecl(hasDirectBase(cxxBaseSpecifier().bind("base")))`

Reviewed By: steveire, aaron.ballman

Differential Revision: https://reviews.llvm.org/D69218

GitOrigin-RevId: 2f181086b5cbbe83c4492aa44484a77ed06ec812
7 files changed