Fix inline builtin handling in case of redefinition

Basically, inline builtin definition are shadowed by externally visible
redefinition. This matches GCC behavior.

The implementation has to workaround the fact that:

1. inline builtin are renamed at callsite during codegen, but
2. they may be shadowed by a later external definition

As a consequence, during codegen, we need to walk redecls and eventually rewrite
some call sites, which is totally inelegant.

Differential Revision: https://reviews.llvm.org/D112059