[clang] [clang-repl] Fix PCH with delayed template parsing (PR #103028)

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> > BTW, the delayed template parsing is a deprecated technique. Both clang and MSVC won't enable this after C++20 and we think it is the root of many bugs.
> 
> I agree. It was needed in the past to parse the MSVC stdlib, let's check if we still need it: [root-project/root#16222](https://github.com/root-project/root/pull/16222)

Update on this: testing shows that we don't need it anymore, so we turned it off. I guess we still want to merge the fix proposed in this PR regardless? @ChuanqiXu9 

https://github.com/llvm/llvm-project/pull/103028