[DebugInfo] Refactor .debug_macro checks. NFCI

Move the Dwarf version checks that determine if the .debug_macro section
should be emitted, into a DwarfDebug member. This is a preparatory
refactoring for allowing the GNU .debug_macro extension, which is a
precursor to the DWARF 5 format, to be emitted by LLVM for earlier DWARF
versions.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D82971