Rollup merge of #156281 - nikic:nofree, r=RalfJung

Emit nofree attribute

Treat the semantics of pointee.size as "dereferenceable-at-point" and always specify the size. Instead, use a separate NoFree attribute to determine whether dereferenceability extends to the whole function.

Then in the LLVM backend, only actually emit dereferenceable if nofree is also set, as dereferenceable currently implies nofree.

In addition, explicitly emit the nofree attribute, which will help when LLVM switches dereferenceable to be at-point.

Relevant recent LLVM PR: llvm/llvm-project#195658

Author: JonathanBrouwer