[MERGED] Improvements for detection of unused assignments

[Daniel-Cortez]

What this PR does / why we need it:

This PR does the following:

• Makes the compiler detect unused assignments that happen after the point where the variable is used:

main()
{
    new n = 1;
    printf("%d", n);
    n = 2; // warning 204: symbol is assigned a value that is never used: "n"
}

• Adds warning 240 (previously assigned value is never used).

new x = 1;
x = 2; // warning 240: previously assigned value is never used (symbol "x")

Which issue(s) this PR fixes:

Fixes #479

What kind of pull this is:

• A Bug Fix
• A New Feature
• Some repository meta (documentation, etc)
• Other

Additional Documentation:

Currently warning 240 doesn't work on arrays, as it would require a lot more modifications to track assignments to each array element individually.
It is also worth noting that some of the code parts in this PR were copied from the implementation of warning 210, as both of them work by the same principle.

[Y-Less]

I've not read the code fully. Does this handle operator=? There should probably be no warning in that case, since you are technically calling a function, which may have desirable side-effects.

[Daniel-Cortez]

Not sure what you mean exactly. Can you give an example so I could check it (and see what I can do about it, if necessary)?

[Y-Less]

Tag:operator=(Tag:a)
{
    printf("hi");
    return Tag:(_:a * 2);
}

main()
{
    new Tag:a = 5, Tag:b;
    // Unused assignment that does something.
    b = a;
}

[Daniel-Cortez]

Done, overloaded assignments are now handled without printing warning 204.

[Daniel-Cortez]

• Rebased to resolve merge conflicts.
• Improved detection of unused assignments for function arguments:

DoSomething(arg) // Now 'arg' is marked as `uASSIGNED` ("assigned a value
{                // that is not used yet") upon the beginning of function body.
    arg = 1; // warning 240: previously assigned value is never used
             // (previously the compiler couldn't detect this).
    #pragma unused arg
}