[BOLT][DWARF] Keep original line info for unmodified units

Summary:
Some compilation units will contain only code that is left unmodified by
BOLT, e.g. there is no profile data available for any function from such
units as they are rarely or never executed.

To save processing time and memory, we disable building line info tables
for such units and write unmodified tables to the output file.

(cherry picked from FBD31599759)