Fix(spark): unqualify columns in PIVOT expressions

[georgesittas]

Spark doesn't seem to allow qualified columns inside of the PIVOT operator's "field" arg (i.e. FOR .. IN (...)):

spark-sql (default)> WITH x(a, b) AS (SELECT * FROM VALUES (1, 2)) SELECT * FROM x PIVOT (SUM(a) FOR b IN (2 AS col));
col
1
Time taken: 0.411 seconds, Fetched 1 row(s)
spark-sql (default)> WITH x(a, b) AS (SELECT * FROM VALUES (1, 2)) SELECT * FROM x PIVOT (SUM(x.a) FOR x.b IN (2 AS col));

[PARSE_SYNTAX_ERROR] Syntax error at or near '.'.(line 1, pos 83)

== SQL ==
WITH x(a, b) AS (SELECT * FROM VALUES (1, 2)) SELECT * FROM x PIVOT (SUM(x.a) FOR x.b IN (2 AS col))
-----------------------------------------------------------------------------------^^^

spark-sql (default)> WITH x(a, b) AS (SELECT * FROM VALUES (1, 2)) SELECT * FROM x PIVOT (SUM(x.a) FOR b IN (2 AS col));
col
1
Time taken: 0.52 seconds, Fetched 1 row(s)

This may be a bug in Spark.