[docs] Add the description about mixing use of clang modules and c++ modules

[ChuanqiXu]

Summary

From the discussion in https://discourse.llvm.org/t/how-should-we-support-dependency-scanner-for-c-20-modules/66027, we get a consensus that we want to support clang modules and c++ modules at the same time. This patch documents this intention.

[ChuanqiXu]

I want to land this two weeks later if no objection shows.

[bruno]

Looks great, some minor suggestions.

clang/docs/StandardCPlusPlusModules.rst

877

I would rewrite the section name as "Interoperability with Clang Modules"

881

"... same time, but the mixed ..."

882

How about: "Please file new github issues as you find interoperability problems"

[vsapsai]

clang/docs/StandardCPlusPlusModules.rst

880

@Bigcheese Is it worth adding something like "A big user of such mode is Objective-C++ code"?

[ChuanqiXu]

Address comments.

clang/docs/StandardCPlusPlusModules.rst

 But we could still save the time for optimizations after IPO and the whole backend.
 
 Overall, at ``O0`` the implementations of functions defined in a module will not impact module users,
 but at higher optimization levels the definitions of such functions are provided to user compilations for the
 purposes of optimization (but definitions of these functions are still not included in the use's object file)-
 this means the build speedup at higher optimization levels may be lower than expected given ``O0`` experience, 
 but does provide by more optimization opportunities.
 
+Interoperability with Clang Modules
+-----------------------------------
+
+We **wish** to support clang modules and standard c++ modules at the same time,
+but the mixed using form is not well used/tested yet.
+
+Please file new github issues as you find interoperability problems.