CGDebugInfo: At the end of EmitFunctionStart, Initialize PrevLoc to the
location that the next call emitLocation() would default to. Otherwise
setLocation() may wrongly believe that the current source file didn't
change, when in fact it did.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204517 91177308-0d34-0410-b5e6-96231b3b80d8