In degenerate but legal inputs, we can have functions that have no source
locations at all -- all the DebugLocs attached to instructions are empty.
LLVM didn't produce any source location for the function; with this patch
it will at least emit the function-scope source location. Demonstrated by
empty-line-info.ll
The XCOFF test modified has similar symptoms -- with this patch, the size
of the ".dwline" section grows a bit, thus shifting some of the file
internal offsets, which I've updated.
7a930ce327