Expose parseXInDimensionList() to the `DialectAsmParser`

This allows custom types and attribute to parse a dimension list that
isn't necessarily terminated with `xtype`, for example something like:

#tf.shape<4x5>

Differential Revision: https://reviews.llvm.org/D100432

mlir/include/mlir/IR/DialectImplementation.h

@@ -361,6 +361,11 @@ public:
   ///   static-dimension-list ::= (integer `x`)*
   virtual ParseResult parseDimensionList(SmallVectorImpl<int64_t> &dimensions,
                                          bool allowDynamic = true) = 0;
+
+  /// Parse an 'x' token in a dimension list, handling the case where the x is
+  /// juxtaposed with an element type, as in "xf32", leaving the "f32" as the
+  /// next token.
+  virtual ParseResult parseXInDimensionList() = 0;
 };
 
 } // end namespace mlir

mlir/lib/Parser/DialectSymbolParser.cpp

@@ -308,6 +308,10 @@ public:
     return parser.parseDimensionListRanked(dimensions, allowDynamic);
   }
 
+  ParseResult parseXInDimensionList() override {
+    return parser.parseXInDimensionList();
+  }
+
   OptionalParseResult parseOptionalType(Type &result) override {
     return parser.parseOptionalType(result);
   }