[C++20] [Modules] Handle inconsistent deduced function return type from importing modules

Close https://github.com/llvm/llvm-project/issues/78830
Close https://github.com/llvm/llvm-project/issues/60085

The direct reason of the issues is that in a module unit, the return
type of a function is deduced to a concrete type (e.g., int) but in the
other module unit, the return type of the same function is not deduced
yet (e.g, auto). Then when we importing the 2 modules, the later
function is selected but the code generator doesn't know how to generate
the auto type. So here is the crash.

The tricky part here is that, when the ASTReader reads the second
unreduced function, it finds the reading function has the same signature
with the already read deduced one and they have the same ODRHash. So
that the ASTReader skips loading the function body of the unreduced
function then the code generator can't infer the undeduced type like it
usually can. Also this is generally fine for functions without deducing
type since it is sufficient to emit a function call without the function
body.

Also in fact, we've already handled the case that the functon has
deduced type and its deducing state is inconsist in different modules:
https://github.com/llvm/llvm-project/blob/3ea92ea2f9d236569f82825cdba6d59bcc22495c/clang/lib/Serialization/ASTReader.cpp#L9531-L9544
and
https://github.com/llvm/llvm-project/blob/3ea92ea2f9d236569f82825cdba6d59bcc22495c/clang/lib/Serialization/ASTReaderDecl.cpp#L3643-L3647.

We've handled the case:
(1) If we read the undeduced functions first and read the deduced functions
later, the compiler will propagate the deduced type info for redecls in
the end of the reading.
(2) If we read the deduced functions first and read the undeduced
functions later, we will propagae the deduced type info when we **complete
the redecl chain**.

However, in the reporting issues, we're in the second case and
reproducer didn't trigger the action to complete the redecl chain. So
here is the crash.

Then it is obvious how should fix the problem. We should complete the
redecl chain for undeduced function types in the end of the reading for
the second case.