The parser rule for expressions in a udf parameter list contains 
two hacks: 
First, the parser input stream is read verbatim, bypassing 
the lexer.
Second, the Item::name field is overwritten. If the argument to a
udf was a field, the field's name as seen by name resolution was
overwritten this way.
If the field name was quoted or escaped, it would appear as e.g. "`field`".
Fixed by not overwriting field names.

mysql-test/r/udf.result:
  Bug#46259: Test result.
mysql-test/t/udf.test:
  Bug#46259: Test case.
sql/sql_yacc.yy:
  Bug#46259: Fix.