Write only minimal .debug_line information.

Summary:
We used to output .debug_line information for every instruction, but because of the way
gdb (and probably lldb as of llvm::DWARFDebugLine::LineTable::findAddress) queries the
line table it's not necessary to output information for two instructions if they follow
each other and map to the same source line. By not repeating this information we generate
a bit less .debug_line data.

(cherry picked from FBD3056402)