[DWARF] DWARFDie::getDeclFile() doesn't work with DW_FORM_implicit_const

DWARFDie::getDeclFile() gets an invalid DWARFFormValue without a DWARFUnit* as DWARFAbbreviationDeclaration::getAttributeValueFromOffset() doesn't pass it in DWARFFormValue::createFromSValue().
Can be checked with DWARFDie::dump(). It works correctly as it gets a referenced value, not a lazy copy.
getDeclFile in DWARFDieTest.cpp has a value stored in DW_FORM_data1, so it passes.

Start with DWARFAbbreviationDeclaration::getAttributeValueFromOffset() and DWARFFormValue::createFromSValue(), then inspect DWARFDie::getDeclFile() and DWARFDie::dump(). Review the getDeclFile test in DWARFDieTest.cpp, which currently uses DW_FORM_data1, and add coverage for DW_FORM_implicit_const. Done means getDeclFile() produces a valid value for that form and DWARFDie::dump() handles it correctly.

Written by the indexing model from the issue text.