cmd/compile: eliminate format string FmtUnsigned support

Passes toolstash -cmp.

Change-Id: I678fc40c0f2a6e9a434bcdd4ea17bb7f319a6063
Reviewed-on: https://go-review.googlesource.com/38144
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/fmt.go

@@ -16,14 +16,11 @@ import (
 // See the respective function's documentation for details.
 type FmtFlag int
 
-// TODO(gri) The ' ' flag is not used anymore in %-formats.
-//           Eliminate eventually.
-
 const ( //                                 fmt.Format flag/prec or verb
 	FmtLeft     FmtFlag = 1 << iota // '-'
 	FmtSharp                        // '#'
 	FmtSign                         // '+'
-	FmtUnsigned                     // ' '               (historic: u flag)
+	FmtUnsigned                     // internal use only (historic: u flag)
 	FmtShort                        // verb == 'S'       (historic: h flag)
 	FmtLong                         // verb == 'L'       (historic: l flag)
 	FmtComma                        // '.' (== hasPrec)  (historic: , flag)
@@ -44,7 +41,7 @@ func fmtFlag(s fmt.State, verb rune) FmtFlag {
 		flag |= FmtSign
 	}
 	if s.Flag(' ') {
-		flag |= FmtUnsigned
+		Fatalf("FmtUnsigned in format string")
 	}
 	if _, ok := s.Precision(); ok {
 		flag |= FmtComma
@@ -1701,7 +1698,6 @@ func fldconv(f *Field, flag FmtFlag) string {
 
 // "%L"  print definition, not name
 // "%S"  omit 'func' and receiver from function types, short type names
-// "% v" package name, not prefix (FTypeId mode, sticky)
 func (t *Type) Format(s fmt.State, verb rune) {
 	switch verb {
 	case 'v', 'S', 'L':