Allow the GNU .debug_macro extension to be emitted for DWARF versions
earlier than 5. The extension is basically what became DWARF 5's format,
except that a DW_AT_GNU_macros attribute is emitted, and some entries
like the strx entries are missing. In this patch I emit GNU's indirect
entries, which are the same as DWARF 5's strp entries.
This patch adds the extension behind a hidden LLVM flag,
-use-gnu-debug-macro. If this can be landed, I would later want to
enable it by default when tuning for GDB and targeting DWARF versions
earlier than 5.
The size of a Clang 8.0 binary built with RelWithDebInfo and the flags
"-gdwarf-4 -fdebug-macro" reduces from 1533 MB to 1349 MB with
.debug_macro (compared to 1296 MB without -fdebug-macro).
One thing that I have not looked into yet is what to do in the DWO case
(when using -gsplit-dwarf). We can't use relocations in the
.debug_macro.dwo section, so the indirect strp entries are not viable