[clang] [llvm] Fix Windows EH IP2State tables (remove +1 bias) (PR #144745)

[via llvm-commits]

namazso wrote:

> For an SEH exception that occurs within a function due to a CALL (such as a bogus indirect call), the IP should point to the beginning of the CALL instruction, so the IP2State tables should point to the correct region.

Yes, I was just pointing out that same-frame catch isn't supposed to be supported in LLVM per the docs. By that rule, this code:

```
void test1(void(*a)()) {
    __try {
        a();
    } __except(1) {
        printf("throw in call!\n");
    }
}

void test2(void(*a)()) {
    __try {
        test1(a);
    } __except (1) {
        printf("throw from call!\n");
    }
}

void int3() {
    __debugbreak();
}

int main() {
    test2(&int3);
    test2((void(*)())0x8FFFFFFFFFFFFFFF);
    return 0;
}
```

Should result in this:

```
throw in call!
throw from call!
```

Which indeed would probably work like that if the start labels were also offset by 1 all the time. Of course it's way different from MSVC, but it sounded at least *consistent* even if slightly insane. However when I tested it on the current LLVM release, it was not consistent at all already, so that's why I commented to just disregard this observation - it already didn't conform whatever the docs were saying, it's not going to be any more broken that it currently is.

>  I'd be happy to improve the state of things for SEH, as well as ensuring that this PR doesn't regress support for GNU's EH support on Windows. If GNU's C++ EH support is based on SEH, then I think I will need to change this PR so that a CALL aligned to .seh_startepilogue always inserts a NOP call. That's easy to do. Do you have pointers to any info on GNU C++ exception dispatch?

Sorry, no clue on GNU EH, my observations were purely about the current state of SEH and the docs (incorrectly) describing the "rules" on how exactly it is partially supported.

https://github.com/llvm/llvm-project/pull/144745