Do not allow variables to be self-assigned through initialization

[Daniel-Cortez]

What this PR does / why we need it:

Fixes variables being able to be self-assigned through initialization expressions (#420).
Example:

main()
{
	// Before the fix: no error
	// After: error 017: undefined symbol "test"
	new test = test;
}

The problem is that the compiler creates symbol test before generating the code for its initialization expression, so test can be used in that expression.
This PR simply makes the compiler temporarily mark the newly created symbol as undefined (until the code for the initialization expression is emitted) and issue an error if the symbol for a local variable is undefined.

Which issue(s) this PR fixes:

Fixes #420

What kind of pull this is:

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

Additional Documentation: