Reduce the number of indirections in the attributes implementation.

- Coallocate entires for AttributeSetImpls and Nodes after the class itself.
- Remove mutable iterators from immutable classes.
- Remove unused context field from AttributeImpl.
- Derive Enum/Align/String attribute implementations from AttributeImpl instead
  of having a whole new inheritance tree for them.
- Derive AlignAttributeImpl from EnumAttributeImpl.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186075 91177308-0d34-0410-b5e6-96231b3b80d8