Fix edge case in 'findBindingAtPosition' when looking up global binding at start of file

[JohnnyMorganz]

The 'findBindingAtPosition' AstQuery function can be used to lookup a local or global binding.

Inside of this function is a check to "Ignore this binding if we're inside its definition. e.g. local abc = abc -- Will take the definition of abc from outer scope".

However, this check is incorrect when we are looking up a global binding at the start of a file.

Consider a complete file with the contents:

local x = stri|ng.char(1)

and we pass the location of the marker | as the position to the find binding position. We will pick up the global binding of the definition string coming from a builtin source (either defined via C++ code or a definitions file and loaded into the global scope).

The global binding string will have a zero position: 0,0,0,0. However, the findBindingLocalStatement check works by looking up the AstAncestry at the binding's defined begin position in the current source module. This will then incorrectly return the local statement for local x, as that is at the start of the source code. Then in turn, we assume we are in the local abc = abc case, and end up skipping over the correct binding.

We fix this by checking if the binding is at the global position. If so, we early exit because it is impossible for a global binding to be defined in a local statement.

[aatxe]

This needs to be flagged.

Suggested change

	if (binding.location == Location{{0, 0}, {0, 0}})
	if (FFlag::LuauFixBindingForGlobalPos && binding.location == Location{{0, 0}, {0, 0}})

(and an associated LUAU_FASTFLAG(LuauFixBindingForGlobalPos) at the top of the file).