[llgo] set debuglocs for calls in synthetic functions

Synthesized functions do not have valid locations,
and so we were not setting a debug location. Recent
changes to DI require that function call instructions
within a function having DI must have a location set.
We just set the debug location to line=0, col=0.

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D22905

llvm-svn: 288772