cmd/compile/internal/types: eliminate FieldName

This functionality can be implemented in package types without a
trampoline back to gc.

Change-Id: Iaff7169fece35482e654553bf16b07dc67d1991a
Reviewed-on: https://go-review.googlesource.com/41416
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -386,9 +386,6 @@ func Main(archInit func(*Arch)) {
 	types.FormatType = func(t *types.Type, s fmt.State, verb rune, mode int) {
 		typeFormat(t, s, verb, fmtMode(mode))
 	}
-	types.FieldName = func(f *types.Field) string {
-		return f.Sym.Name
-	}
 	types.TypeLinkSym = func(t *types.Type) *obj.LSym {
 		return typenamesym(t).Linksym()
 	}

src/cmd/compile/internal/types/type.go

@@ -1221,7 +1221,7 @@ func (t *Type) FieldOff(i int) int64 {
 	return t.Field(i).Offset
 }
 func (t *Type) FieldName(i int) string {
-	return FieldName(t.Field(i))
+	return t.Field(i).Sym.Name
 }
 
 func (t *Type) NumElem() int64 {

src/cmd/compile/internal/types/utils.go

@@ -25,7 +25,6 @@ var (
 	Tconv       func(*Type, int, int, int) string // orig: func tconv(t *Type, flag FmtFlag, mode fmtMode, depth int) string
 	FormatSym   func(*Sym, fmt.State, rune, int)  // orig: func symFormat(sym *Sym, s fmt.State, verb rune, mode fmtMode)
 	FormatType  func(*Type, fmt.State, rune, int) // orig: func typeFormat(t *Type, s fmt.State, verb rune, mode fmtMode)
-	FieldName   func(*Field) string
 	TypeLinkSym func(*Type) *obj.LSym
 	Ctxt        *obj.Link