[clang] [NFC] Move PaddingClearer functionality into ASTContext (PR #215823)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > We actually want access to this computation from Sema as well. See #200362 . Maybe we just want to put this API on ASTContext and cache the result.
> 
> Oh, interesting! I see that the complaint there is that it doesn't do ... something-about-unions correctly. BUT I can just extract this to ASTContext. @cor3ntin and @AaronBallman said it wasn't worth moving all the way up there, but I now see that it does!

Yeah, I was trying to see if we had a need for this to be a more generic visitor and the only uses I found were in codegen, so good catch.

https://github.com/llvm/llvm-project/pull/215823