[DebugInfo] Improve new line printing in debug line verbose output

The new line printing for debug line verbose output was inconsistent.
For new rows in the matrix, a blank line followed, whilst the
DW_LNS_copy opcode actually resulted in two blank lines. There was also
potential inconsistency in the blank lines at the end of the table. This
patch mostly resolves these issues - no blank lines appear in the output
except for a single line after the prologue and at table end to separate
it from any subsquent table, plus some instances after error messages.

Also add a unit test for verbose output to test the fine details of new
line placement and other aspects of verbose output.

Reviewed by: dblaikie

Differential Revision: https://reviews.llvm.org/D81102