Assert that DebugLoc is valid

[davide]

Summary

Noticed when trying to fix PR23380.
I think it's better to assert than segfaulting one line below.

[davide]

Discussed with dexonsmith on IRC.

lib/IR/DiagnosticInfo.cpp

 bool DiagnosticInfoOptimizationBase::isLocationAvailable() const {
   return getDebugLoc();
 }
 
 void DiagnosticInfoOptimizationBase::getLocation(StringRef *Filename,
                                                  unsigned *Line,
                                                  unsigned *Column) const {
   DILocation *L = getDebugLoc();
+  assert(L != nullptr && "debug location is invalid");
   *Filename = L->getFilename();
   *Line = L->getLine();
   *Column = L->getColumn();
 }
 
 const std::string DiagnosticInfoOptimizationBase::getLocationStr() const {
   StringRef Filename("<unknown>");
   unsigned Line = 0;