[CodeComplete] Avoid name clashes of 'Kind' inside CodeCompletionContext. NFC

CodeCompletionContext had declarations of field and enum inside, both named 'Kind'.
It caused gcc 4.8 to give an incorrent warning when refering to enum as
`enum CodeCompletionContext::Kind`.

Avoid that warning by renaming the private field to CCKind.

llvm-svn: 325496