[DWARF] Set discriminator to 0 for DW_LNS_copy

Summary:
Make DW_LNS_copy set the discriminator register to 0, to conform to
DWARF 4 & 5: "Then it sets the discriminator register to 0, and sets the
basic_block, prologue_end and epilogue_begin registers to false."

Because all of DW_LNE_end_sequence, DN_LNS_copy, and special opcodes reset
discriminator to 0, we can move discriminator=0 to appendRowToMatrix.

Also, make DW_LNS_copy print before appending the row, as it is similar
to a address+=0,line+=0 special opcode, which prints before appending
the row.

Reviewers: dblaikie, probinson, aprantl

Reviewed By: dblaikie

Subscribers: danielcdh, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60364

llvm-svn: 358148