[SPARK-20924][SQL] Unable to call the function registered in the not-current database

### What changes were proposed in this pull request?
We are unable to call the function registered in the not-current database.
```Scala
sql("CREATE DATABASE dAtABaSe1")
sql(s"CREATE FUNCTION dAtABaSe1.test_avg AS '${classOf[GenericUDAFAverage].getName}'")
sql("SELECT dAtABaSe1.test_avg(1)")
```
The above code returns an error:
```
Undefined function: 'dAtABaSe1.test_avg'. This function is neither a registered temporary function nor a permanent function registered in the database 'default'.; line 1 pos 7
```

This PR is to fix the above issue.
### How was this patch tested?
Added test cases.

Author: Xiao Li <gatorsmile@gmail.com>

Closes #18146 from gatorsmile/qualifiedFunction.