[ODRHash diagnostics] Split `err_module_odr_violation_mismatch_decl_diff` into...

[ODRHash diagnostics] Split `err_module_odr_violation_mismatch_decl_diff` into per-entity diagnostics. NFC.

We'll need to add more cases for Objective-C entities and adding
everything to `err_module_odr_violation_mismatch_decl_diff` makes it
harder to work with over time.

Differential Revision: https://reviews.llvm.org/D128488