gccrs: Generate backend drop flags for conditional moves

[Lishin1215]

This patch connects the conditional BIR Drop analysis to the existing backend cleanup.

For now, in a conditional move case like:

fn f(cond: bool) {
    let x = Droppable { value: 1 };


    if cond {
        let y = x;
    }
}

The BIR Drop analysis classifies the cleanup of x as:

Drop(x): Conditional

The backend then creates a Drop flag for x.
Conceptually, the generated backend code behaves like:

try {
    bool flag = false;
    let x = Droppable { value: 1 };
    flag = true;


    if (cond) {
        let y = x;
        flag = false;
    }
}
finally {
    if (flag) {
        flag = false;
        Drop(x);
    }
}

• When x is initialized, the flag is set to true.
• When x is moved, the flag is cleared to false, so Drop(x) is skipped during cleanup.
• When x is not moved, the flag remains true, so Drop(x) is called.

[Lishin1215]

I added conditionally_dropped to save those that are classified as Conditional.
I need this information to determine whether a variable requires a Drop flag.
It is used when generating the backend code.

[Lishin1215]

I added move_sources to map the move expression HirId to the source local HirId.

Since the move expression x and the source local x have different HirIds.
And also I need the source local x's HirId to find the Drop flag,
so this map lets me find the source HirId from the move expression HirId.

For example:

...
  let x = Droppable();  // source local HirId = 10
  x.flag = true;  // associate with source local HirId 10
...
  let y = x;  // move expression HirId = 30

Here I only have HirId 30, but the flag is associated with HirId 10.
So I need:

  move_sources[30] = 10

Then the backend can use HirId 10 to find the correct flag and clear it:

x.flag = false;