Implement code completion for tags, e.g., code completion after "enum"

will provide the names of various enumerations currently
visible. Introduced filtering of code-completion results when we build
the result set, so that we can identify just the kinds of declarations
we want.

This implementation is incomplete for C++, since we don't consider
that the token after the tag keyword could start a
nested-name-specifier. 

llvm-svn: 82222