[ClangRepl] Reland Semanic Code Completion (#75556)

This patch contains changes from
002d471a, in
addition to a bug fix that added a virtual destructor to
`CompletionContextHandler`

The original changes in the orginal commit piggybacks on clang's
semantic modules to enable semantic completion.  In particular, we use
`CodeCompletionContext` to differentiate two types of code completion.
We also
extract the relevant type information from it.