MCParser/Debug info: Accept line number 0 as a legitimate value, since
CFE produces it to indicate artificial locations.
c.f.: DWARF standard, Table 6.2:
line -- An unsigned integer indicating a source line number. Lines are numbered beginning at 1. The compiler may emit the value 0 in cases where an instruction cannot be attributed to any source line.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191471 91177308-0d34-0410-b5e6-96231b3b80d8