[SPARK-17753][SQL] Allow a complex expression as the input a value based case statement

## What changes were proposed in this pull request?
We currently only allow relatively simple expressions as the input for a value based case statement. Expressions like `case (a > 1) or (b = 2) when true then 1 when false then 0 end` currently fail. This PR adds support for such expressions.

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

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

Closes #15322 from hvanhovell/SPARK-17753.

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

@@ -527,16 +527,16 @@ valueExpression
     ;
 
 primaryExpression
-    : constant                                                                                 #constantDefault
-    | name=(CURRENT_DATE | CURRENT_TIMESTAMP)                                                  #timeFunctionCall
+    : name=(CURRENT_DATE | CURRENT_TIMESTAMP)                                                  #timeFunctionCall
+    | CASE value=expression whenClause+ (ELSE elseExpression=expression)? END                  #simpleCase
+    | CASE whenClause+ (ELSE elseExpression=expression)? END                                   #searchedCase
+    | CAST '(' expression AS dataType ')'                                                      #cast
+    | constant                                                                                 #constantDefault
     | ASTERISK                                                                                 #star
     | qualifiedName '.' ASTERISK                                                               #star
     | '(' expression (',' expression)+ ')'                                                     #rowConstructor
-    | qualifiedName '(' (setQuantifier? expression (',' expression)*)? ')' (OVER windowSpec)?  #functionCall
     | '(' query ')'                                                                            #subqueryExpression
-    | CASE valueExpression whenClause+ (ELSE elseExpression=expression)? END                   #simpleCase
-    | CASE whenClause+ (ELSE elseExpression=expression)? END                                   #searchedCase
-    | CAST '(' expression AS dataType ')'                                                      #cast
+    | qualifiedName '(' (setQuantifier? expression (',' expression)*)? ')' (OVER windowSpec)?  #functionCall
     | value=primaryExpression '[' index=valueExpression ']'                                    #subscript
     | identifier                                                                               #columnReference
     | base=primaryExpression '.' fieldName=identifier                                          #dereference

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala

@@ -1138,7 +1138,7 @@ class AstBuilder extends SqlBaseBaseVisitor[AnyRef] with Logging {
    * }}}
    */
   override def visitSimpleCase(ctx: SimpleCaseContext): Expression = withOrigin(ctx) {
-    val e = expression(ctx.valueExpression)
+    val e = expression(ctx.value)
     val branches = ctx.whenClause.asScala.map { wCtx =>
       (EqualTo(e, expression(wCtx.condition)), expression(wCtx.result))
     }

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

@@ -292,6 +292,10 @@ class ExpressionParserSuite extends PlanTest {
   test("case when") {
     assertEqual("case a when 1 then b when 2 then c else d end",
       CaseKeyWhen('a, Seq(1, 'b, 2, 'c, 'd)))
+    assertEqual("case (a or b) when true then c when false then d else e end",
+      CaseKeyWhen('a || 'b, Seq(true, 'c, false, 'd, 'e)))
+    assertEqual("case 'a'='a' when true then 1 end",
+      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))
   }