Background: The current spec is imprecise with respect to the parsing ambiguity
for composite literals: It says that the ambiguity arises when the TypeName form
of the LiteralType is used. The following code:

    if (B) {} ...

is not using the TypeName form (but the parenthesized TypeName form) and thus
could be interpreted as:

    if ((B){}) ...

instead of

    if B {} ...

Both compilers and gofmt choose the latter interpretation. One could fix the
spec by making the clause regarding the parsing ambiguity more precise ("...using
the _possibly parenthesized_ TypeName form of the LiteralType..."). The alternative
(chosen here) is to simply disallow parenthesized literal types. Except for a single
test case (test/parentype.go) there appears to be no Go code under $GOROOT containing
parenthesized literal types. Furthermore, parentheses are never needed around a
literal type for correct parsing.

R=golang-dev
CC=golang-dev
https://golang.org/cl/1913041