Set end position for implicit class declaration tree part of a compact source file

[shivam71]

Issue : End position not found for implicit class scaffolding compact source file (JEP 512)

Context

• Compact source file allow top level methods and fields.
• Internally the fields and methods are treated as part of a implicit final class.
• As the class in not explicitly declared in code the IDE when trying to find end position for the corresponding java element fails.
• This change defaults to using the end position of the compilation unit tree (which is computed correctly) when for a class element source position is found but no end position is found.
• Visible effect of this issue was missing outline view for the compact source file in the Java VSCode Extension.

Debugging output for reference

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