Update FieldTarget.getBaseType()
Update FieldTarget.getBaseType()

It wasn't defined when `getBase()` was an EvalImplicitDerefInstruction.
Rewriting it like this means it should work no matter what type of
instruction `getBase()` is.

github/codeql,github/codeql,github/codeql-go,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql-go,github/codeql,github/codeql,github/codeql,github/codeql-go,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql-go