Eliminate cursor kinds used to express definitions. Instead, provide

CIndex functions that (1) map from a reference or declaration to the
corresponding definition, if available, and (2) determine whether a
given declaration cursor is also a definition. This eliminates a lot
of duplication in the cursor kinds, and maps more closely to the Clang
ASTs.

This is another API + ABI breaker with no deprecation. Yay, progress.

llvm-svn: 93893