Change MCExpr::EvaluateAsRelocatableImpl of variables to return the original

variable if recursing fails to simplify it.

Factor AliasedSymbol to be a method of MCSymbol.

Update MCAssembler::EvaluateFixup to match the change in
EvaluateAsRelocatableImpl.

Remove the WeakRefExpr hack, as the object writer now sees the weakref with
no extra effort needed.

Nothing else is using MCTargetExpr, but keep it for now.

Now that the ELF writer sees relocations with aliases, handle

    .weak    foo2
foo2:
    .weak    bar2
    .set    bar2,foo2
    .quad    bar2

the same way gas does and produce a relocation with bar2.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119152 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCSymbol.h

@@ -138,6 +138,11 @@ namespace llvm {
       return Value;
     }
 
+    // AliasedSymbol() - If this is an alias (a = b), return the symbol
+    // we ultimately point to. For a non alias, this just returns the symbol
+    // itself.
+    const MCSymbol &AliasedSymbol() const;
+
     void setVariableValue(const MCExpr *Value);
 
     /// @}