[clang] [Clang][NFC] Use const ASTContext reference in Decl Create methods (PR #185401)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

@AaronBallman should take a look at this.  I don't have a problem with it, but I'm suspicious of it, as using the ASTContext for allocation SEEMS like it shouldn't be a 'const' thing to do .

https://github.com/llvm/llvm-project/pull/185401