Fix CPU fallback for Map lookup.

Fixes NVIDIA#5180.

Map lookup is currently supported only in cases where the keys are
scalar values. In case the keys are specified as a vector (e.g.
expressions), the plugin should fall back to CPU.
NVIDIA#4944 introduced a bug in how literal signatures are specified for
multiple data types. This breaks CPU fallback.

This commit fixes the specification of literals-only `TypeSig`.

sql-plugin/src/main/scala/com/nvidia/spark/rapids/TypeChecks.scala

@@ -181,6 +181,17 @@ final class TypeSig private(
     new TypeSig(it, maxAllowedDecimalPrecision, childTypes, lt, notes)
   }
 
+  /**
+   * Add a literal restriction to the signature
+   * @param dataTypes the types that have to be literal. Will be added if they do not already exist.
+   * @return the new signature.
+   */
+  def withLit(dataTypes: TypeEnum.ValueSet): TypeSig = {
+    val it = initialTypes ++ dataTypes
+    val lt = litOnlyTypes ++ dataTypes
+    new TypeSig(it, maxAllowedDecimalPrecision, childTypes, lt, notes)
+  }
+
   /**
    * All currently supported types can only be literal values.
    * @return the new signature.
@@ -531,7 +542,7 @@ object TypeSig {
    * Create a TypeSig that only supports literals of certain given types.
    */
   def lit(dataTypes: TypeEnum.ValueSet): TypeSig =
-    new TypeSig(dataTypes)
+    TypeSig.none.withLit(dataTypes)
 
   /**
    * Create a TypeSig that supports only literals of common primitive CUDF types.

sql-plugin/src/main/scala/org/apache/spark/sql/rapids/complexTypeExtractors.scala

@@ -254,10 +254,10 @@ case class GpuGetMapValue(child: Expression, key: Expression, failOnError: Boole
   }
 
   override def doColumnar(lhs: GpuScalar, rhs: GpuColumnVector): ColumnVector =
-    throw new IllegalStateException("This is not supported yet")
+    throw new IllegalStateException("Map lookup keys must be scalar values")
 
   override def doColumnar(lhs: GpuColumnVector, rhs: GpuColumnVector): ColumnVector =
-    throw new IllegalStateException("This is not supported yet")
+    throw new IllegalStateException("Map lookup keys must be scalar values")
 
   override def left: Expression = child