Add a getSymbolOffset method and use it in the ELF writer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120977 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCAsmLayout.h

@@ -133,6 +133,10 @@ public:
   /// layout.
   uint64_t getSymbolAddress(const MCSymbolData *SD) const;
 
+  /// \brief Get the offset of the given symbol, as computed in the current
+  /// layout.
+  uint64_t getSymbolOffset(const MCSymbolData *SD) const;
+
   /// @}
 };