[CodeView] Initialize column-end to zero

CodeView, unlike DWARF, can associate code with a range of columns.
However, LLVM can only represent a single column position internally.

We used to claim that the end column and start column were the same
which yielded less than satisfactory results: we would stop printing at
the _beginning_ of the source expression!  Instead, mark the column-end
as 'zero' to indicate that we don't have one (as per the documentation
for IDiaLineNumber::get_lineNumberEnd).

llvm-svn: 257528