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 issue 9001.

LGTM=iant, r, rsc
R=r, rsc, iant, ken, dave
CC=golang-codereviews
https://codereview.appspot.com/160570043