[ODRHash][NFC] Add missing 'select' case for `ODRMismatchDecl`.

[vsapsai]

Summary

No test changes because err_module_odr_violation_mismatch_decl_unknown
is a catch-all when custom diagnostic is missing. And missing custom
diagnostic we should fix by implementing it, not by improving the
general case. But if we pass enum value not covered by 'select', clang
can crash, so protect against that.

[vsapsai]

We emit err_module_odr_violation_mismatch_decl_unknown at
https://github.com/llvm/llvm-project/blob/0fbe3f3f486e01448121f7931a4ca29fac1504ab/clang/lib/Serialization/ASTReader.cpp#L11143-L11150

ODRMismatchDecl is defined at
https://github.com/llvm/llvm-project/blob/0fbe3f3f486e01448121f7931a4ca29fac1504ab/clang/lib/Serialization/ASTReader.cpp#L9624-L9638

[rsmith]

We perhaps can't or don't want to add test coverage for the "unexpected decl" case here. Can we add coverage for the "function template" case?

[vsapsai]

To trigger "function template|different function template" diagnostic we need the entire "case FunctionTemplate" in "switch (FirstDiffType)" not to detect any mismatches. And I don't know any such cases.

[vsapsai]

For the record, FunctionTemplate case was added in https://reviews.llvm.org/rG9359e8f22a5403ad9524a92c4ccab4db46c9c100

[rsmith]

OK, fair enough.

[vsapsai]

Thanks for the review!

clang/include/clang/Basic/DiagnosticSerializationKinds.td

def note_module_odr_violation_enum : Note<"but in '%0' found "
   "%ordinal2 element %3 %select{has|does not have}4 an initializer|"
   "%ordinal2 element %3 has different initializer|"
   "}1">;
 
 def err_module_odr_violation_mismatch_decl_unknown : Error<
   "%q0 %select{with definition in module '%2'|defined here}1 has different "
   "definitions in different modules; first difference is this "
   "%select{||||static assert|field|method|type alias|typedef|data member|"
-  "friend declaration|unexpected decl}3">;
+  "friend declaration|function template|"
+  "unexpected decl}3">;
 def note_module_odr_violation_mismatch_decl_unknown : Note<
   "but in '%0' found "
   "%select{||||different static assert|different field|different method|"
   "different type alias|different typedef|different data member|"
-  "different friend declaration|another unexpected decl}1">;
+  "different friend declaration|different function template|"
+  "another unexpected decl}1">;
 
 def warn_duplicate_module_file_extension : Warning<
   "duplicate module file extension block name '%0'">,
   InGroup<ModuleFileExtension>;
 
 def warn_module_system_bit_conflict : Warning<
   "module file '%0' was validated as a system module and is now being imported "
   "as a non-system module; any difference in diagnostic options will be ignored">,