[DebugInfo] Simplify DwarfDebug::emitMacro

Broken out from a review comment on D82975. This is an NFC expect for
that the Macinfo macro string is now emitted using a single emitBytes()
invocation, so it can be done using a single string directive.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D83557