[DWARF parser] Use enums instead of bitfields in DILineInfoSpecifier.

It is more appropriate than the current situation, when one flag
(AbsoluteFilePath) is relevant only if another flag is set.

This refactoring would also simplify fetching the short function name
(stored in DW_AT_name) instead of a linkage name returned currently.

No functionality change.

llvm-svn: 208921