Add DWARF5 variants of assembly scan tests that use DW_FORM_implicit_const
Some DWARF tests scan the assembly output looking for constant values.
When using DWARF5 those constants might use DW_FORM_implicit_const,
which are output (in the comments) after the attribute instead of
before. To make sure these tests work introduce a -gdwarf-5 variant
of these tests and explicitly use -gdwarf-2 for the original.