Distinguish between attributes explicitly written at the request of the user, and attributes implicitly generated to assist in bookkeeping by the compiler. This is done so by table generating a CreateImplicit method for each attribute.

Additionally, remove the optional nature of the spelling list index when creating attributes. This is supported by table generating a Spelling enumeration when the spellings for an attribute are distinct enough to warrant it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199378 91177308-0d34-0410-b5e6-96231b3b80d8