[ObjectYAML] Fix issue with DWARF2 AddrSize 8

In my refactoring I introduced a bug where we were using the reference size instead of the offset size for DW_FORM_strp and similar forms.

This patch resolves the error and adds a test case testing all the DWARF forms for DWARF2 AddrSize 8. There is similar coverage already in the DWARFDebugInfoTest sources that covers the parser. Once I migrate the DWARFGenerator APIs to be built on the YAML tools they will be fully covered under the same tests.

llvm-svn: 297230