add empty var ( ), type ( ), const ( ) factorings

suggested by rob (could make programs generating
go easier to write).

R=ken
DELTA=13  (13 added, 0 deleted, 0 changed)
OCL=16766
CL=16766

src/cmd/gc/go.y

@@ -117,6 +117,7 @@ imports:
 import:
 	LIMPORT import_stmt
 |	LIMPORT '(' import_stmt_list_r osemi ')'
+|	LIMPORT '(' ')'
 
 import_stmt:
 	import_here import_package import_there
@@ -223,6 +224,10 @@ Acommon_dcl:
 	{
 		$$ = rev($3);
 	}
+|	LVAR '(' ')'
+	{
+		$$ = N;
+	}
 |	LCONST '(' constdcl osemi ')'
 	{
 		iota = 0;
@@ -235,6 +240,10 @@ Acommon_dcl:
 		lastconst = N;
 		$$ = N;
 	}
+|	LCONST '(' ')'
+	{
+		$$ = N;
+	}
 |	LTYPE Atypedcl
 	{
 		$$ = N;
@@ -243,6 +252,10 @@ Acommon_dcl:
 	{
 		$$ = N;
 	}
+|	LTYPE '(' ')'
+	{
+		$$ = N;
+	}
 
 Bcommon_dcl:
 	LVAR Bvardcl