[clang][codegen] Hoist parameter attribute setting in function prolog.

Summary:
- If the coerced type is still a pointer, it should be set with proper
  parameter attributes, such as `noalias`, `nonnull`, and etc. Hoist
  that (pointer) parameter attribute setting so that the coerced pointer
  parameter could be marked properly.

Depends on D79394

Reviewers: rjmccall, kerbowa, yaxunl

Subscribers: jvesely, nhaehnle, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79395