[DebugLine] save one debug line entry for empty prologue

Some debuggers like DBX on AIX assume the address in debug line
entries is always incremental. But clang generates two entries (entry
for file scope line and entry for prologue end) with same address if
prologue is empty

And if the prologue is empty, seems the first debug line entry for the
function is unnecessary(i.e. removing the first entry won't impact the
behavior in GDB on Linux), so I implement this for all debuggers.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D147506