[SPARK-3973] Print call site information for broadcasts

[shivaram]

Its hard to debug which broadcast variables refer to what in a big codebase. Printing call site information helps in debugging.

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[shivaram]

Jenkins, retest this please

[SparkQA]

QA tests have started for PR 2829 at commit cd6dbdf.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2829 at commit cd6dbdf.

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

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[shivaram]

@rxin Can you take a look ?

[andrewor14]

LGTM

[JoshRosen]

LGTM, so I've merged this. Thanks!