[clang] [Clang] Move DeclContext::getEnclosingFunction/castEnclosingFunction out of line (PR #224711)

[Ambrose Leeb via cfe-commits]

https://github.com/Sirraide approved this pull request.

Right, of course FunctionDecl wouldn't be complete in DeclBase.h; that makes sense...

https://github.com/llvm/llvm-project/pull/224711