[llvm] Add support for the .base64 directive (fixes #165499) (PR #165549)

[Ben Kallus via llvm-commits]

kenballus wrote:

> You probably need to merge `main` into your branch to get the tests to pass, I'm not sure though. The failures are unrelated as far as I can tell.
> 
> Can you also check your code formatting again? The comment above has been re-used to show the diff for where things need to be updated.

Got it. Now merged and reformatted.

https://github.com/llvm/llvm-project/pull/165549