Fix DWARFDie::getDeclFile(...) to work with DW_AT_specification.

DWARFDie::getDeclFile(...) previously only supported getting the DW_AT_decl_file if the DIE itself contained the DW_AT_decl_file attribute, or if the DIE had a DW_AT_abstract_origin that pointed to another DIE that had a DW_AT_decl_file. This patch allows the function to get the right attribute value if there is a DW_AT_specification that points to another DIE. We also test that if a DW_AT_abtract_origin or DW_AT_specification points to a DIE in another CU with a DW_FORM_ref_addr, that the right line table is used to extract the file index.

Full tests were added for the following cases:
- DIE has a DW_AT_decl_file attribute
- DIE has a DW_AT_abtract_origin that points to another die in the same CU
- DIE has a DW_AT_abtract_origin that points to another die in another CU
- DIE has a DW_AT_specification that points to another die in the same CU
- DIE has a DW_AT_specification that points to another die in another CU

Differential Revision: https://reviews.llvm.org/D108480