[clang] [ClangRepl] Reland Semanic Code Completion (PR #75556)

[Fred Fu via cfe-commits]

capfredf wrote:

@vgvassilev the commit message in the original commit isn't very useful. To my recollection, I wrote more detailed message than `Differential Revision: https://reviews.llvm.org/D159128`, but I can't find it now. The original development branch has been deleted. So I rewrote the message. 

https://github.com/llvm/llvm-project/pull/75556