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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 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

At least it is good for the pre-c++20 codes : )

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