- Generate __data_start symbol that marks the start of .data when __data_start
  is referenced from code being linked.
- add .gnu.warning.SYMBOL support to ld.lld(1)

Index: tools/lld/ELF/Symbols.h
--- tools/lld/ELF/Symbols.h.orig
+++ tools/lld/ELF/Symbols.h
@@ -142,6 +142,9 @@ class Symbol { (public)
   // True if this symbol is specified by --trace-symbol option.
   uint8_t traced : 1;
 
+  // True if the .gnu.warning.SYMBOL is set for the symbol
+  uint8_t gwarn : 1;
+
   inline void replace(const Symbol &newSym);
 
   bool includeInDynsym() const;
@@ -247,7 +250,7 @@ class Symbol { (public)
         type(type), stOther(stOther), symbolKind(k), visibility(stOther & 3),
         isUsedInRegularObj(!file || file->kind() == InputFile::ObjKind),
         exportDynamic(isExportDynamic(k, visibility)), inDynamicList(false),
-        canInline(false), referenced(false), traced(false), needsPltAddr(false),
+        canInline(false), referenced(false), traced(false), gwarn(false), needsPltAddr(false),
         isInIplt(false), gotInIgot(false), isPreemptible(false),
         used(!config->gcSections), needsTocRestore(false),
         scriptDefined(false) {}
@@ -436,6 +439,9 @@ struct ElfSym {
   // __bss_start
   static Defined *bss;
 
+  // __data_start
+  static Defined *data;
+
   // etext and _etext
   static Defined *etext1;
   static Defined *etext2;
@@ -557,6 +563,7 @@ void Symbol::replace(const Symbol &newSym) {
   canInline = old.canInline;
   referenced = old.referenced;
   traced = old.traced;
+  gwarn = old.gwarn;
   isPreemptible = old.isPreemptible;
   scriptDefined = old.scriptDefined;
   partition = old.partition;
@@ -575,6 +582,8 @@ void Symbol::replace(const Symbol &newSym) {
 void maybeWarnUnorderableSymbol(const Symbol *sym);
 bool computeIsPreemptible(const Symbol &sym);
 void reportBackrefs();
+
+extern llvm::DenseMap<StringRef, StringRef> gnuWarnings;
 
 // A mapping from a symbol to an InputFile referencing it backward. Used by
 // --warn-backrefs.