[C++20] [Modules] [Reduced BMI] Handling Deduction Guide in reduced BMI
carefully

Close https://github.com/llvm/llvm-project/issues/93859

The direct pattern of the issue is that, in a reduced BMI, we're going
to wrtie a class but we didn't write the deduction guide. Although we
handled deduction guide, but we tried to record the found deduction
guide from `noload_lookup` directly.

It is slightly problematic if the found deduction guide is from AST.
e.g.,

```
module;
export module m;
import xxx; // Also contains the class and the deduction guide
...
```

Then when we writes the class in the current file, we tried to record
the deduction guide, but `noload_lookup` returns the deduction guide
from the AST file then we didn't record the local deduction guide. Then
mismatch happens.

To mitiagte the problem, we tried to record the canonical declaration
for the decution guide.