[clang] [Clang][NFC] Remove TemplateArgumentList::OnStack (PR #79760)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@erichkeane I ended up writing a python script that compiles the TUs in `clang/lib` with `-print-stats` and then sums up the total bytes allocated by `ASTContext`. Ended up with a 0.4% decrease in memory usage.

This does however rely on `compile_commands.json`, and boost does not have a great way to generate this for every subproject... Do you think the results from compiling clang are sufficient? 

https://github.com/llvm/llvm-project/pull/79760