[SPARK-5040][SQL] Support expressing unresolved attributes using $"attribute name" notation in SQL DSL.

[rxin]

No description provided.

[SparkQA]

Test build #24970 has started for PR 3862 at commit 72121af.

• This patch merges cleanly.

[rxin]

This subsumes #3649

[SparkQA]

Test build #24970 has finished for PR 3862 at commit 72121af.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • implicit class StrongToAttributeConversionHelper(val sc: StringContext)

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[culler]

Hi @rxin I think you meant to name the class StringToAttributeConversionHelper.

[rxin]

Thanks, @culler. Fixed.

[SparkQA]

Test build #24975 has started for PR 3862 at commit 9b10f57.

• This patch merges cleanly.

[SparkQA]

Test build #24975 has finished for PR 3862 at commit 9b10f57.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • implicit class StringToAttributeConversionHelper(val sc: StringContext)

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/24975/
Test PASSed.

[rxin]

cc @marmbrus

[marmbrus]

LGTM

[marmbrus]

Thanks! Merged to master.