Move MCSymbol Value in to the union of Offset and CommonSize.

This is a reapplication of r239440 which was reverted in r239441.
There are no changes to this patch from then, but this had instead exposed
a bug in .thumb_set which was fixed in r240318.  Having fixed that bug, it
is now safe to re-apply this code.

Original commit message below:

It wasn't possible to have a variable Symbol with offset or 'isCommon' so
this just enables better packing of the MCSymbol class.

Reviewed by Rafael Espindola.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240320 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCSymbol.cpp

@@ -40,7 +40,11 @@ void *MCSymbol::operator new(size_t s, const StringMapEntry<bool> *Name,
 void MCSymbol::setVariableValue(const MCExpr *Value) {
   assert(!IsUsed && "Cannot set a variable that has already been used.");
   assert(Value && "Invalid variable value!");
+  assert((SymbolContents == SymContentsUnset ||
+          SymbolContents == SymContentsVariable) &&
+         "Cannot give common/offset symbol a variable value");
   this->Value = Value;
+  SymbolContents = SymContentsVariable;
   SectionOrFragment = nullptr;
 }