Distinguish between attributes explicitly written at the request of the user,...

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.

llvm-svn: 199378