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

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