[SPARK-22153][SQL] Rename ShuffleExchange -> ShuffleExchangeExec

[rxin]

What changes were proposed in this pull request?

For some reason when we added the Exec suffix to all physical operators, we missed this one. I was looking for this physical operator today and couldn't find it, because I was looking for ExchangeExec.

How was this patch tested?

This is a simple rename and should be covered by existing tests.

[gatorsmile]

LGTM

[SparkQA]

Test build #82265 has finished for PR 19376 at commit 1de6165.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #82266 has finished for PR 19376 at commit 4d08353.

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

[gatorsmile]

retest this please

[SparkQA]

Test build #82271 has finished for PR 19376 at commit 4d08353.

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

[gatorsmile]

Thanks! Merged to master.