[DWARF][MC] improve the error message when DwarfLineStrSection is null.

[Esme]

Summary

Currently the crush info is ambiguous when DwarfLineStrSection is null.
The patch adds an assertion in the constructor of MCDwarfLineStr when DwarfLineStrSection is null.

llvm/lib/MC/MCDwarf.cpp

if (AddrDelta % MinInsnLength != 0) {
     // TODO: report this error, but really only once.
     ;
   }
   return AddrDelta / MinInsnLength;
 }
 
 MCDwarfLineStr::MCDwarfLineStr(MCContext &Ctx) {
   UseRelocs = Ctx.getAsmInfo()->doesDwarfUseRelocationsAcrossSections();
-  if (UseRelocs)
-    LineStrLabel =
-        Ctx.getObjectFileInfo()->getDwarfLineStrSection()->getBeginSymbol();
+  if (UseRelocs) {
+    MCSection *DwarfLineStrSection =
+        Ctx.getObjectFileInfo()->getDwarfLineStrSection();
+    assert(DwarfLineStrSection && "DwarfLineStrSection must not be NULL");
+    LineStrLabel = DwarfLineStrSection->getBeginSymbol();
+  }
 }
 
 //
 // This is called when an instruction is assembled into the specified section
 // and if there is information from the last .loc directive that has yet to have
 // a line entry made for it is made.
 //
 void MCDwarfLineEntry::make(MCStreamer *MCOS, MCSection *Section) {