Reland "[clang-repl] support code completion at a REPL."

Original commit message:
"
This patch enabled code completion for ClangREPL. The feature was built upon
three existing Clang components: a list completer for LineEditor, a
CompletionConsumer from SemaCodeCompletion, and the ASTUnit::codeComplete method.
The first component serves as the main entry point of handling interactive inputs.

Because a completion point for a compiler instance has to be unchanged once it
is set, an incremental compiler instance is created for each code
completion. Such a compiler instance carries over AST context source from the
main interpreter compiler in order to obtain declarations or bindings from
previous input in the same REPL session.

The most important API codeComplete in Interpreter/CodeCompletion is a thin
wrapper that calls with ASTUnit::codeComplete with necessary arguments, such as
a code completion point and a ReplCompletionConsumer, which communicates
completion results from SemaCodeCompletion back to the l...