[SPARK-30431][SQL] Update SqlBase.g4 to create commentSpec pattern like locationSpec

[yaooqinn]

What changes were proposed in this pull request?

In SqlBase.g4, the comment clause is used as COMMENT comment=STRING and COMMENT STRING in many places.

While the location clause often appears along with the comment clause with a pattern defined as

locationSpec
    : LOCATION STRING
    ;

Then, we have to visit locationSpec as a List but comment as a single token.

We defined commentSpec for the comment clause to simplify and unify the grammar and the invocations.

Why are the changes needed?

To simplify the grammar.

Does this PR introduce any user-facing change?

no

How was this patch tested?

existing tests

[SparkQA]

Test build #116147 has finished for PR 27102 at commit 177cc7b.

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• This patch adds no public classes.

[SparkQA]

Test build #116151 has finished for PR 27102 at commit 755a80b.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #116156 has finished for PR 27102 at commit d68f83e.

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

[yaooqinn]

Cc @cloud-fan thanks

[SparkQA]

Test build #116202 has finished for PR 27102 at commit d8d85e8.

• This patch fails to build.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #116204 has finished for PR 27102 at commit fbe36f3.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #116210 has finished for PR 27102 at commit e3de92c.

• This patch fails due to an unknown error code, -9.
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• This patch adds no public classes.

[SparkQA]

Test build #116212 has finished for PR 27102 at commit bd5544b.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[yaooqinn]

retest this please

[SparkQA]

Test build #116229 has finished for PR 27102 at commit bd5544b.

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

[cloud-fan]

thanks, merging to master!