MC: introduce ability to restrict recorded relocations

Add support to allow a target specific COFF object writer to restrict the
recorded resolutions in the emitted object files.  This is motivated by the need
in Windows on ARM, where an intermediate relocation needs to be prevented from
being emitted in the object file.

llvm-svn: 209336

llvm/include/llvm/MC/MCWinCOFFObjectWriter.h

@@ -30,6 +30,7 @@ namespace llvm {
     virtual unsigned getRelocType(const MCValue &Target,
                                   const MCFixup &Fixup,
                                   bool IsCrossSection) const = 0;
+    virtual bool recordRelocation(const MCFixup &) const { return true; }
   };
 
   /// \brief Construct a new Win COFF writer instance.

llvm/lib/MC/WinCOFFObjectWriter.cpp

@@ -808,6 +808,7 @@ void WinCOFFObjectWriter::RecordRelocation(const MCAssembler &Asm,
     }
   }
 
+  if (TargetObjectWriter->recordRelocation(Fixup))
     coff_section->Relocations.push_back(Reloc);
 }