cmd/compile/internal/syntax: correct an error string

When we have an error in a function type used in an expression
we don't know until we see an opening { whether we have a function
literal or a function type. Use "function type" as context because
that's always correct in the specific error message.

Change-Id: I9aad8fcddf31ae53daa53cebd2c2001f08eabde0
Reviewed-on: https://go-review.googlesource.com/c/go/+/401316
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/syntax/parser.go

@@ -991,7 +991,7 @@ func (p *parser) operand(keep_parens bool) Expr {
 	case _Func:
 		pos := p.pos()
 		p.next()
-		_, ftyp := p.funcType("function literal")
+		_, ftyp := p.funcType("function type")
 		if p.tok == _Lbrace {
 			p.xnest++