Remove `Ignore` flag from SymbolAssignment class.

Previously, Ignore flag is set if we don't want to assign
a value to symbols. It happens if a symbol assingment is in
PROVIDE() and there's already a symbol with the same name.

The previous code had a subtle but that we assume that the
existing symbol is an absolute symbol even if it is not.
This patch fixes the issue by always overwriting an absolute
symbol.

llvm-svn: 277115