[Support][ErrorOr] Add optimized specialization of ErrorOr<void>.

ErrorOr<void> represents an operation that returns nothing, but can still fail.
It should be used in cases where you need the aditional user data that ErrorOr
provides over error_code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173209 91177308-0d34-0410-b5e6-96231b3b80d8
Use the AttributeSet when adding multiple attributes and an Attribute::AttrKind
when adding a single attribute to the function.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173210 91177308-0d34-0410-b5e6-96231b3b80d8
[Support][ErrorOr] Don't use nullptr :(

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173212 91177308-0d34-0410-b5e6-96231b3b80d8
Use the AttributeSet when removing multiple attributes. Use Attribute::AttrKind
when removing one attribute. This further encapsulates the use of the attributes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173214 91177308-0d34-0410-b5e6-96231b3b80d8