[ODRHash diagnostics] Move repetetive code at lambda calls into lambdas themselves. NFC.

It helps to avoid copy-paste mistakes and makes custom code paths more
noticeable.

Not funnelling all diagnostic through `ODRDiagDeclError` because plan to
break down `err_module_odr_violation_mismatch_decl_diff` into smaller
pieces instead of making it bigger and bigger.

Differential Revision: https://reviews.llvm.org/D128487