[SPARK-21957][SQL][FOLLOWUP] Support CURRENT_USER without tailing parentheses

[yaooqinn]

What changes were proposed in this pull request?

A followup for 345d35e, in this PR we support CURRENT_USER without tailing parentheses in default mode. And for ANSI mode, we can only use CURRENT_USER without tailing parentheses because it is a reserved keyword that cannot be used as a function name

Why are the changes needed?

1. make it the same as current_date/current_timestamp
2. better ANSI compliance

Does this PR introduce any user-facing change?

no, just a followup

How was this patch tested?

new tests

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43813/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43813/

[SparkQA]

Test build #139289 has finished for PR 32770 at commit 25df635.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[yaooqinn]

cc @cloud-fan

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43845/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43845/

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43852/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43852/

[SparkQA]

Test build #139323 has finished for PR 32770 at commit d567bbe.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #139330 has finished for PR 32770 at commit 6e4b021.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

thanks, merging to master!