[BOLT][DWARF] Fix for .debug_line with DWARF5

There was a bug in a code that pre-populated line string for a case where parts
of .debug_line are not processed by BOLT, but copied as raw data. We were not
switching sections. This resulted in parts of the binary being over-written with
debug data.

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D154544