[clang] [clang] On-demand parsing for templated and internal-linkage functions (PR #210893)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

I think we need to figure out how to scope the changes so they don't land intrusively in clang. One way to move forward is to see if we can limit this to interpreter-only (eg IncrementalExtensions=OnDemandParsingX). I don't think we should touch the compiler everywhere at that stage.

https://github.com/llvm/llvm-project/pull/210893