autofix: discard captures + some multi for loop support

[travisstaloch]

this patch makes autofix add discards for unused loop and switch case captures which have curlies. it prevents adding duplicate 'remove capture' actions by checking previous action ranges.

it removes special casing of index captures now that multi for loops have arrived.

• make getCaptureLoc() return only single Loc
• remove CaptureLocs struct which is no longer used
• add DiagnosticKind.@"switch tag capture" to handle 'inline else => |x, tag|' discards
• add test "getCaptureLoc"

[travisstaloch]

addresses #689

tested with

test {
    for (0..10, 0..10, 0..10) |i, j, k| {}
    switch (union(enum) {}{}) {
        inline .a => |cap, tag| {},
    }
    if (null) |x| {}
    if (null) |v| {} else |e| {}
    _ = null catch |e| {};
}

this becomes:

test {
    for (0..10, 0..10, 0..10) |i, j, k| {
        _ = k;
        _ = j;
        _ = i;
    }
    switch (union(enum) {}{}) {
        inline .a => |cap, tag| {
            _ = tag;
            _ = cap;
        },
    }
    if (null) |x| {
        _ = x;
    }
    if (null) |v| {
        _ = v;
    } else |e| {
        _ = e;
    }
    _ = null catch |e| {
        _ = e;
    };
}

[leecannon]

This looks good 👍

[travisstaloch]

interesting note: just tested a release-fast build and the discard order is reversed back to source order.

test {
    for (0..10, 0..10, 0..10) |i, j, k| {
        _ = i;
        _ = j;
        _ = k;
    }
...

glad to see they're in source order. not sure why they were reversed in the debug build.