[llvm] [PAC] Make ValueMapper handle ConstantPtrAuth values (PR #129088)

[Anton Korobeynikov via llvm-commits]

asl wrote:

I already happen to have this patch locally, but for `bugpoint` / `llvm-reduce` purposes.

However, it turned out that that it is required in LTO as mapper is used there. When assertions are enabled we're seeing assert being triggered at https://github.com/llvm/llvm-project/pull/129088/files#diff-8f546b2d38006292655c6fd7287a101d758f8f12be381f0180354431e387d3ceR540

But, in the release build w/o assert we're ending with constant zero instead producing the wrong code. The reproducer is as easy as:
```c++
// m.h:
// A caller may actually call this
void m2Export(const char* input);

// Called by m2
int export_foo(const char* str);

// Only called within m1
__attribute__((noinline)) int foo(int(*printfn)(const char* const, ...), const char* str);
int bar(const char* const, ...);

// m1.c
#include "m.h"

__attribute__((noinline)) int foo(int(*printfn)(const char* const, ...), const char* str) {
    printfn("Called foo");
    printfn(str);
    return 0;
}

int export_foo(const char* str) {
    return foo(bar, str);
}

// m2.c
#include <stdio.h>

#include "m.h"

void m2Export(const char* input) {
    printf("I will call export_foo\n");
    export_foo(input);
    printf("I have called export_foo\n");
}

// m3.c
#include <stdio.h>

int bar(const char* const str, ...) {
    printf("Bar is called with input: ");
    printf("%s", str);
    return 0;
}
```

`m2Export` (m2.c) calls `export_foo` (m1.c) which calls `foo` (m1.c, noinline attribute) which passes a function pointer argument `bar` (m3.c)

`export_foo` is inlined and instead `foo` is called directly, however instead of `bar`'s address, a `nullptr` would be passed

https://github.com/llvm/llvm-project/pull/129088