[clang] [NFC][clang][AST] Add compile-time dispatch for random access iterators in append() (PR #162000)

[Samarth Narang via cfe-commits]

snarang181 wrote:

Been a while since this has been up for review. Even though @zwuis has approved it, they'd be more comfortable with another review. Thanks. 

https://github.com/llvm/llvm-project/pull/162000