[DebugInfo] Handle fixed-width DW_FORM_addrx variants in DWARFFormValue::getAsSectionedAddress()

Previously this would incorrectly return the raw offset into the .debug_addr section for the
DW_FORM_addrx1/2/3/4 forms rather than the actual address.

Note that this was handled correctly in the dump() function so this issue only occurs for users
of this API and not in tools such as llvm-dwarfdump. The dump() method has now been updated
to use this method to increase coverage.

This also now adds a few unit tests for indexed addresses to DWARFDebugInfoTest.

Differential Revision: https://reviews.llvm.org/D143073