[mlir] fix a crash if the dialect is missing a data layout interface

[ftynse]

Summary

The top-level verifier of data layout specifications delegates verification of
entries with identifier keys to the dialect of the identifier prefix. This flow
was missing a check whether the dialect actually implements the relevant
interface.

[gysit]

Looks good to me!

mlir/lib/Interfaces/DataLayoutInterfaces.cpp

for (const auto &kvp : ids) {
 
     // Ignore attributes that belong to an unknown dialect, the dialect may
     // actually implement the relevant interface but we don't know about that.
     if (!dialect)
       continue;
 
     const auto *iface =
         dialect->getRegisteredInterface<DataLayoutDialectInterface>();
+    if (!iface) {
+      return emitError(loc)
+             << "the '" << dialect->getNamespace()
+             << "' dialect does not support identifier data layout entries";
+    }
     if (failed(iface->verifyEntry(kvp.second, loc)))
       return failure();
   }
 
   return success();
 }
 
 #include "mlir/Interfaces/DataLayoutAttrInterface.cpp.inc"
 #include "mlir/Interfaces/DataLayoutOpInterface.cpp.inc"
 #include "mlir/Interfaces/DataLayoutTypeInterface.cpp.inc"

mlir/test/Interfaces/DataLayoutInterfaces/types.mlir

 // RUN: mlir-opt --split-input-file --verify-diagnostics --test-data-layout-query %s | FileCheck %s
 
 // expected-error@below {{expected integer attribute in the data layout entry for 'index'}}
 module attributes { dlti.dl_spec = #dlti.dl_spec<
   #dlti.dl_entry<index, [32]>>} {
 }
 
 // -----
 
+// expected-error@below {{the 'test' dialect does not support identifier data layout entries}}
+"test.op_with_data_layout"() { dlti.dl_spec = #dlti.dl_spec<
+  #dlti.dl_entry<index, 32>,
+  #dlti.dl_entry<"test.foo", [32]>>} : () -> ()
+
+// -----
+
 // CHECK-LABEL: @index
 module @index attributes { dlti.dl_spec = #dlti.dl_spec<
   #dlti.dl_entry<index, 32>>} {
   func @query() {
     // CHECK: bitsize = 32
     "test.data_layout_query"() : () -> index
     return
   }