Unordered directives

[delcypher]

We need to add support for checking un-ordered directives. For example the order of error messages produced by a tool might not be predictable.

The proposal is to add a CHECK-BEGIN-UNORDERED and CHECK-END-UNORDERED directives which would be used as follows

// CHECK-L: <This must come 1st>
// CHECK-L: <This must come 2nd>

// CHECK-BEGIN-UNORDERED

// CHECK-L: <This can be 3rd or 4th>
// CHECK-L: <This can be 3rd or 4th>

// CHECK-END-UNORDERED

// CHECK-L: <This must be 5th>

Unfortunately implementing support for this isn't completely trivial. But hopefully I'll get around to it at some point.