[clang] [clang-tools-extra] [lldb] [Clang][AST][NFC] (`RecordDecl` -> `CXXRecordDecl`)`::isInjectedClassName` (PR #148195)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov approved this pull request.

LGTM

For reference, this takes this change out of https://github.com/llvm/llvm-project/pull/147835, which reduces the size of that PR a little bit.

https://github.com/llvm/llvm-project/pull/148195