[ODRHash diagnostics] Preparation to minimize subsequent diffs. NFC.

Specifically, making the following changes:
* Turn lambdas calculating ODR hashes into static functions.
* Move `ODRCXXRecordDifference` where it is used.
* Rename some variables and move some lines of code.
* Replace `auto` with explicit type when the deduced type is not mentioned.
* Add `const` for unmodified objects, so we can pass them to more functions.

Differential Revision: https://reviews.llvm.org/D128690