Change prototype merging error into a warning for builtins

As was observed in https://reviews.llvm.org/D123627#3707635, it's
confusing that a user can write:
```
float rintf(void) {}
```
and get a warning, but writing:
```
float rintf() {}
```
gives an error. This patch changes the behavior so that both are
warnings, so that users who have functions which conflict with a
builtin identifier can still use that identifier as they wish.

Differential Revision: https://reviews.llvm.org/D131499

(cherry picked from commit 4c02ab8c)