spec: permit parentheses around builtin function names

Not a language change.

This is simply documenting the status quo which permits
builtin function names to be parenthesized in calls; e.g.,
both

        len(s)
and
        (((len)))(s)

are accepted by all compilers and go/types.

Changed the grammar by merging the details of BuiltinCall
with ordinary Calls. Also renamed the Call production to
Arguments which more clearly identifies that part of the
grammar and also matches better with its counterpart on
the declaration side (Parameters).

The fact that the first argument can be a type (for builtins)
or cannot be a type (for regular function calls) is expressed
in the prose, no need to make the grammar more complicated.

Fixes #9001.

LGTM=iant, r, rsc
R=r, rsc, iant, ken, dave
CC=golang-codereviews
https://golang.org/cl/160570043

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of October 23, 2014",
+	"Subtitle": "Version of October 27, 2014",
 	"Path": "/ref/spec"
 }-->
 
@@ -2449,12 +2449,11 @@ Primary expressions are the operands for unary and binary expressions.
 PrimaryExpr =
 	Operand |
 	Conversion |
-	BuiltinCall |
 	PrimaryExpr Selector |
 	PrimaryExpr Index |
 	PrimaryExpr Slice |
 	PrimaryExpr TypeAssertion |
-	PrimaryExpr Call .
+	PrimaryExpr Arguments .
 
 Selector       = "." identifier .
 Index          = "[" Expression "]" .
@@ -2462,8 +2461,7 @@ Slice          = "[" ( [ Expression ] ":" [ Expression ] ) |
                      ( [ Expression ] ":" Expression ":" Expression )
                  "]" .
 TypeAssertion  = "." "(" Type ")" .
-Call           = "(" [ ArgumentList [ "," ] ] ")" .
-ArgumentList   = ExpressionList [ "..." ] .
+Arguments      = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
 </pre>
 
 
@@ -5371,11 +5369,6 @@ so they can only appear in <a href="#Calls">call expressions</a>;
 they cannot be used as function values.
 </p>
 
-<pre class="ebnf">
-BuiltinCall = identifier "(" [ BuiltinArgs [ "," ] ] ")" .
-BuiltinArgs = Type [ "," ArgumentList ] | ArgumentList .
-</pre>
-
 <h3 id="Close">Close</h3>
 
 <p>