[clang][CodeGen] Avoid emitting ifuncs with undefined resolvers

[clang][CodeGen] Avoid emitting ifuncs with undefined resolvers

The purpose of this change is to fix the following codegen bug:

```
// main.c
__attribute__((cpu_specific(generic)))
int *foo(void) { static int z; return &z;}
int main() { return *foo() = 5; }

// other.c
__attribute__((cpu_dispatch(generic))) int *foo(void);

// run:
clang main.c other.c -o main; ./main
```

This will segfault prior to the change, and return the correct
exit code 5 after the change.

The underlying cause is that when a translation unit contains
a cpu_specific function without the corresponding cpu_dispatch
the generated code binds the reference to foo() against a
GlobalIFunc whose resolver is undefined. This is invalid: the
resolver must be defined in the same translation unit as the
ifunc, but historically the LLVM bitcode verifier did not check
that. The generated code then binds against the resolver rather
than the ifunc, so it ends up calling the resolver rather than
the resolvee. In the example above it treats its return value as
an int *, therefore trying to write to program text.

The root issue at the representation level is that GlobalIFunc,
like GlobalAlias, does not support a "declaration" state. The
object which provides the correct semantics in these cases
is a Function declaration, but unlike Functions, changing a
declaration to a definition in the GlobalIFunc case constitutes
a change of the object type, as opposed to simply emitting code
into a Function.

I think this limitation is unlikely to change, so I implemented
the fix by returning a function declaration rather than an ifunc
when encountering cpu_specific, and upgrading it to an ifunc
when emitting cpu_dispatch.
This uses `takeName` + `replaceAllUsesWith` in similar vein to
other places where the correct IR object type cannot be known
locally/up-front, like in `CodeGenModule::EmitAliasDefinition`.

Previous discussion in: https://reviews.llvm.org/D112349

Signed-off-by: Itay Bookstein <ibookstein@gmail.com>
Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D120266