[clang] [clang][test] add TestLanguage.def to specify all tested language versions (PR #94243)

[Julian Schmidt via cfe-commits]

5chmidti wrote:

rebase + ping. @AaronBallman could you check if this is okay now? I could merge the AST matcher doc testing stack if that is the case

https://github.com/llvm/llvm-project/pull/94243