[mlir] Do not set lastToken in AsmParser's resetToken function

The member lastToken is the last token that was consumed by the parser.
Resetting the lexer position to a different position does not cause any token to
be consumed, so lastToken should not be updated. Setting it to curToken can
cause the scopeLoc.end location of OperationDefinition to be off-by-one, pointing to the
first token after the operation.

An example for an operation for which the scopeLoc.end location was wrong before is:
%0 = torch.vtensor.literal(dense_resource<__elided__> : tensor<768xbf16>)
 : !torch.vtensor<[768],bf16> . Here the scope end loc always pointed to the next token

Author: jorickert

mlir/lib/AsmParser/Parser.h

@@ -130,10 +130,14 @@ class Parser {
     consumeToken();
   }
 
-  /// Reset the parser to the given lexer position.
+  /// Reset the parser to the given lexer position. Resetting the parser/lexer 
+  /// position does not update 'state.lastToken'. 'state.lastToken' is the 
+  /// last parsed token, and is used to provide the scope end location for
+  /// OperationDefinitions. To ensure the correctness of the end location, the  
+  /// last consumed token of an OperationDefinition needs to be the last token
+  /// belonging to it.
   void resetToken(const char *tokPos) {
     state.lex.resetPointer(tokPos);
-    state.lastToken = state.curToken;
     state.curToken = state.lex.lexToken();
   }