Follow-up to r205999: Emit an artificial location (valid scope, line 0)
for CXXGlobalInit/Dtor helper functions.
This makes _GLOBAL__I_a regain its DW_AT_high/low_pc in the debug info.

Thanks to echristo for catching this!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206088 91177308-0d34-0410-b5e6-96231b3b80d8