[LinkerScript] Change ErrorOr usage to fix MSVC2012 buildbots

Number parsing functions used an ErrorOr<> idiom that is not supported in
MSVC2012. This patch fixes this.


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221128 91177308-0d34-0410-b5e6-96231b3b80d8