Add r224985 back with fixes.

The fixes are to note that AArch64 has additional restrictions on when local
relocations can be used. In particular, ld64 requires that relocations to
cstring/cfstrings use linker visible symbols.

Original message:

In an assembly expression like

bar:
  .long L0 + 1

the intended semantics is that bar will contain a pointer one byte past L0.

In sections that are merged by content (strings, 4 byte constants, etc), a
single position in the section doesn't give the linker enough information.
For example, it would not be able to tell a relocation must point to the
end of a string, since that would look just like the start of the next.

The solution used in ELF to use relocation with symbols if there is a non-zero
addend.

In MachO before this patch we would just keep all symbols in some sections.

This would miss some cases (only cstrings on x86_64 were implemented) and was
inefficient since most relocations have an addend of 0 and can be represented
without the symbol.

This patch implements the non-zero addend logic for MachO too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226503 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCAssembler.h

@@ -11,6 +11,7 @@
 #define LLVM_MC_MCASSEMBLER_H
 
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallString.h"
@@ -881,6 +882,8 @@ private:
 
   iplist<MCSymbolData> Symbols;
 
+  DenseSet<const MCSymbol *> LocalsUsedInReloc;
+
   /// The map of sections to their associated assembler backend data.
   //
   // FIXME: Avoid this indirection?
@@ -980,6 +983,9 @@ private:
                                         MCFragment &F, const MCFixup &Fixup);
 
 public:
+  void addLocalUsedInReloc(const MCSymbol &Sym);
+  bool isLocalUsedInReloc(const MCSymbol &Sym) const;
+
   /// Compute the effective fragment size assuming it is laid out at the given
   /// \p SectionAddress and \p FragmentOffset.
   uint64_t computeFragmentSize(const MCAsmLayout &Layout,