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

[Samarth Narang via cfe-commits]

snarang181 wrote:

@AaronBallman -- any guidance on if I should be able to merge this if I receive your approval? All comments have been addressed. 

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