[COFF] more informative "broken object file" diagnostics

[inglorion]

Summary

When a symbol refers to a special section or a section that doesn't
exist, lld would fatal with "broken object file". This change gives a
different message for each scenario, and includes the name of the
file, name of the symbol, and the section being referred to.

[ruiu]

LGTM

[inglorion]

s/existant/existent/

lld/trunk/COFF/InputFiles.cpp

if (Sym.isExternal())
       return Symtab->addAbsolute(Name, Sym);
     else
       return make<DefinedAbsolute>(Name, Sym);
   }
   int32_t SectionNumber = Sym.getSectionNumber();
   if (SectionNumber == llvm::COFF::IMAGE_SYM_DEBUG)
     return nullptr;
 
-  // Reserved sections numbers don't have contents.
   if (llvm::COFF::isReservedSectionNumber(SectionNumber))
-    fatal("broken object file: " + toString(this));
+    fatal(toString(this) + ": " + Name +
+          " should not refer to special section " + Twine(SectionNumber));
 
-  // This symbol references a section which is not present in the section
-  // header.
   if ((uint32_t)SectionNumber >= SparseChunks.size())
-    fatal("broken object file: " + toString(this));
+    fatal(toString(this) + ": " + Name +
+          " should not refer to non-existent section " + Twine(SectionNumber));
 
   // Handle comdat leader symbols.
   if (const coff_aux_section_definition *Def = ComdatDefs[SectionNumber]) {
     ComdatDefs[SectionNumber] = nullptr;
     Symbol *Leader;
     bool Prevailing;
     if (Sym.isExternal()) {
       COFFObj->getSymbolName(Sym, Name);