[SPARK-18214][SQL] Simplify RuntimeReplaceable type coercion

[rxin]

What changes were proposed in this pull request?

RuntimeReplaceable is used to create aliases for expressions, but the way it deals with type coercion is pretty weird (each expression is responsible for how to handle type coercion, which does not obey the normal implicit type cast rules).

This patch simplifies its handling by allowing the analyzer to traverse into the actual expression of a RuntimeReplaceable.

How was this patch tested?

• Correctness should be guaranteed by existing unit tests already
• Removed SQLCompatibilityFunctionSuite and moved it sql-compatibility-functions.sql
• Added a new test case in sql-compatibility-functions.sql for verifying explain behavior.

[marmbrus]

LGTM

[SparkQA]

Test build #67935 has finished for PR 15723 at commit 8cdf56c.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • trait RuntimeReplaceable extends UnaryExpression with Unevaluable
  • case class IfNull(left: Expression, right: Expression, child: Expression)
  • case class NullIf(left: Expression, right: Expression, child: Expression)
  • case class Nvl(left: Expression, right: Expression, child: Expression) extends RuntimeReplaceable
  • case class Nvl2(expr1: Expression, expr2: Expression, expr3: Expression, child: Expression)

[SparkQA]

Test build #67936 has finished for PR 15723 at commit 75e7839.

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

[SparkQA]

Test build #3393 has finished for PR 15723 at commit a5c0b16.

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

[SparkQA]

Test build #67953 has finished for PR 15723 at commit a5c0b16.

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

[SparkQA]

Test build #67961 has finished for PR 15723 at commit 5face4a.

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

[SparkQA]

Test build #67963 has finished for PR 15723 at commit 01cfa00.

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

[rxin]

Merging in master/branch-2.1.