[Support] Remove HashString and replace it with djbHash. NFC.

[JDevlieghere]

Summary

This removes the HashString function from StringExtraces and replaces
its uses with calls to djbHash from DJB.h, also in Support.

[JDevlieghere]

The alternative is to keep HashString and just have it call djbHash.

[aprantl]

Do we need to worry about performance on non-LTO builds because HashString won't be inlined any more?
I'm leaning towards no.

[aprantl]

LGTM, thanks! Perhaps give it another 24 hours to see if anyone objects.

[aprantl]

Also please be sure to not land this before we fixed the implementation of llvm::djbHash() to use unsigned chars or else this isn't NFC.

[JDevlieghere]

In D43615#1015902, @aprantl wrote:

Also please be sure to not land this before we fixed the implementation of llvm::djbHash() to use unsigned chars or else this isn't NFC.

Pavel already landed that change, so we should be good!

[labath]

In D43615#1015902, @aprantl wrote:

Also please be sure to not land this before we fixed the implementation of llvm::djbHash() to use unsigned chars or else this isn't NFC.

The HashString function used a different initial value (0 vs 5381), so this is still not fully NFC, but I hope that does not matter.

include/lld/Core/SymbolTable.h

 //===- Core/SymbolTable.h - Main Symbol Table -----------------------------===//
 //
 //                             The LLVM Linker
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLD_CORE_SYMBOL_TABLE_H
 #define LLD_CORE_SYMBOL_TABLE_H
 
 #include "lld/Common/LLVM.h"
 #include "llvm/ADT/DenseSet.h"
-#include "llvm/ADT/StringExtras.h"
+#include "llvm/Support/DJB.h"
 #include <cstring>
 #include <map>
 #include <vector>
 
 namespace lld {
 
 class AbsoluteAtom;
 class Atom;

 
 private:
   typedef llvm::DenseMap<const Atom *, const Atom *> AtomToAtom;
 
   struct StringRefMappingInfo {
     static StringRef getEmptyKey() { return StringRef(); }
     static StringRef getTombstoneKey() { return StringRef(" ", 1); }
     static unsigned getHashValue(StringRef const val) {
-      return llvm::HashString(val);
+      return llvm::djbHash(val, 0);
     }
     static bool isEqual(StringRef const lhs, StringRef const rhs) {
       return lhs.equals(rhs);
     }
   };
   typedef llvm::DenseMap<StringRef, const Atom *,
                                            StringRefMappingInfo> NameToAtom;