Use a PointerUnion in MCSymbol for Section and Fragment. NFC.

The Fragment and Section, and a bool for HasFragment were all used to create
a PointerUnion.  Just use a pointer union instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239324 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCSymbol.cpp

@@ -42,8 +42,7 @@ void MCSymbol::setVariableValue(const MCExpr *Value) {
   assert(!IsUsed && "Cannot set a variable that has already been used.");
   assert(Value && "Invalid variable value!");
   this->Value = Value;
-  Section = nullptr;
-  HasFragment = false;
+  SectionOrFragment = nullptr;
 }
 
 void MCSymbol::print(raw_ostream &OS) const {