[clang] [C++20][Modules] Load function body from the module that gives canonical decl (PR #111992)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> > Thanks for the analysis. I feel (2) is easier to me IIUC. I feel it is a natural extension of your previous work. WDYT?
> 
> I implemented this approach and it works. I tried to limit eager deserialization to the minimal set of functions (at the moment it is friend inline friend functions). PTAL.

LGTM. And a minor point is, the term "eager deserialization" makes me to think you're going to deserialize the decls eagerly, which made me shocked initially : )

let's wait for @ilya-biryukov and see if he can give a test for this on google's workloads before landing

https://github.com/llvm/llvm-project/pull/111992