Reapplying r204952 a second time.

Clean up the __has_attribute implementation without modifying its behavior.

Replaces the tablegen-driven AttrSpellings.inc, which lived in the lexing layer with AttrHasAttributeImpl.inc, which lives in the basic layer. Updates the preprocessor to call through to this new functionality which can take additional information into account (such as scopes and syntaxes).

Expose the ability for parts of the compiler to ask whether an attribute is supported for a given spelling (including scope), syntax, triple and language options.