ast-gen: use correct token_src for switch, if and while exprs

[travisstaloch]

fixes #18579

i've verified that after this change, the following error ranges are correct and point at the identifiers instead of the asterisks

    switch (undefined) {
        .a => |*ident| {},
        //      ^^^^^ unused capture
    }
    if (undefined) |*foo| {} else |err| {}
    //               ^^^ unused capture
    while (undefined) |*foo| {} else |err| {}
    //                  ^^^ unused capture

[travisstaloch]

I added back in this line per suggestion by @Techatrix. I believe this applies to

    const e: error{A}!u32 = error.A;
    if (e) |*ptr| {...} else |err| switch (err) {...}
    //      ^ previous range
    //       ^^^ current range

EDIT
I've verified this is now correct and was previously broken in 0.12.0-dev.2158+4f2009de1

[travisstaloch]

I think I'm missing some compile error tests. Correct me if I'm wrong, but I think they should go in test/cases/compile_errors. I'm working on adding them now.

[travisstaloch]

I've added the compile error tests. Not sure about this stuff:

// error
// backend=stage2
// target=native
//

I copied that from another file in that directory and they pass locally running $ zig build test-cases.