Fix: added UnionSyntheticProvider for LLDB

[Mottl]

This PR fixes crashing of rust-lldb when union inside union holds variants which are aligned differently.

The following code will crash rust-lldb while inspecting AlignedStruct:

use std::mem::ManuallyDrop;

#[repr(C, align(8))]
struct AlignedStruct {
    data: [u8; 16],
}

union SomeUnion {
    a: (),
    b: ManuallyDrop<AlignedStruct>,
}

fn main() {
    let s = SomeUnion {
        b: ManuallyDrop::new(AlignedStruct {
            data: *b"Hello World\0\0\0\0\0",
        }),
    };
    println!("break here"); // breakpoint here and expand `s`
}

Trying to expand s will crash lldb with SIGSEGV

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

r? @Walnut356, can you take a look and see if this makes sense?

View changes since this review

[Mark-Simulacrum]

Can you say more about this? If LLDB can decide, why can't it always do the right thing? Does this still leave the bug in place for some set of cases?

[rustbot]

Failed to set assignee to Walnut356: invalid assignee

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[Walnut356]

I was not able to replicate the crash. I tried compiling your example program, and for both *-gnu and *-msvc I ran it through

• LLDB 21.1.4 and 21.1.8 with Rust's built-in visualizers (rustc 1.90.0, 1.93.9, and 1.94.0)
• LLDB 21.1.4 and 22.1.2 and with my own custom visualizers
• CodeLLDB 1.12.0 (LLDB version 21.1.7-codelldb, defers to rust's built-in visualizers)

CLI LLDB simply omitted the a field in all cases, CodeLLDB reported it as error: invalid value object for *-gnu targets, but otherwise didn't complain. There were no issues with the b field

If possible, can you provide the following:

• LLDB Version
• rustc version
• target triple you're compiling for
• any debug adapter you are using and its version (e.g. CodeLLDB, rustrover debugger)

[rust-bors]

☔ The latest upstream changes (presumably #154824) made this pull request unmergeable. Please resolve the merge conflicts.