Extract a bc attr parsing helper that returns Attribute::None on error

Extract a bc attr parsing helper that returns Attribute::None on error

The parsing method still returns llvm::error_code for consistency with
other parsing methods.  Minor cleanup, no functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194437 91177308-0d34-0410-b5e6-96231b3b80d8