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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• Owners of files modified in this PR: @Mark-Simulacrum