Move all of the MCSymbol COFF flags logic in to MCSymbolCOFF.

Move all of the MCSymbol COFF flags logic in to MCSymbolCOFF.

All flags setting/getting is now done in the class with helper methods instead
of users having to get the bits in the correct order.

Reviewed by Rafael Espíndola.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239314 91177308-0d34-0410-b5e6-96231b3b80d8