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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@82222 91177308-0d34-0410-b5e6-96231b3b80d8