[SPARK-19472][SQL] Parser should not mistake CASE WHEN(...) for a function call

## What changes were proposed in this pull request?
The SQL parser can mistake a `WHEN (...)` used in `CASE` for a function call. This happens in cases like the following:
```sql
select case when (1) + case when 1 > 0 then 1 else 0 end = 2 then 1 else 0 end
from tb
```
This PR fixes this by re-organizing the case related parsing rules.

## How was this patch tested?
Added a regression test to the `ExpressionParserSuite`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #16821 from hvanhovell/SPARK-19472.

sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4

@@ -530,8 +530,8 @@ valueExpression
 
 primaryExpression
     : name=(CURRENT_DATE | CURRENT_TIMESTAMP)                                                  #timeFunctionCall
-    | CASE value=expression whenClause+ (ELSE elseExpression=expression)? END                  #simpleCase
     | CASE whenClause+ (ELSE elseExpression=expression)? END                                   #searchedCase
+    | CASE value=expression whenClause+ (ELSE elseExpression=expression)? END                  #simpleCase
     | CAST '(' expression AS dataType ')'                                                      #cast
     | constant                                                                                 #constantDefault
     | ASTERISK                                                                                 #star

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala

@@ -298,6 +298,8 @@ class ExpressionParserSuite extends PlanTest {
       CaseKeyWhen("a" ===  "a", Seq(true, 1)))
     assertEqual("case when a = 1 then b when a = 2 then c else d end",
       CaseWhen(Seq(('a === 1, 'b.expr), ('a === 2, 'c.expr)), 'd))
+    assertEqual("case when (1) + case when a > b then c else d end then f else g end",
+      CaseWhen(Seq((Literal(1) + CaseWhen(Seq(('a > 'b, 'c.expr)), 'd.expr), 'f.expr)), 'g))
   }
 
   test("dereference") {