support reversing field names

THey don't show up in stack traces, and if they show up in regular
program output, we should in theory not obfuscate those names via the
detection of reflection.

However, there's one relatively common scenario where obfuscated field
names can appear: in build error messages, when obfuscation fails a
build.

reverse.go

@@ -81,6 +81,10 @@ func commandReverse(args []string) error {
 					addReplace(node.Name.Name)
 				case *ast.TypeSpec:
 					addReplace(node.Name.Name)
+				case *ast.Field:
+					for _, name := range node.Names {
+						addReplace(name.Name)
+					}
 
 				case *ast.CallExpr:
 					// continues below

testdata/scripts/reverse.txt

@@ -12,12 +12,20 @@ cp stderr main.stderr
 # This output is not reproducible between 'go test' runs,
 # so we can't use a static golden file.
 grep 'goroutine 1 \[running\]' main.stderr
-! grep 'ExportedLibFunc|unexportedMainFunc|test/main|main\.go|lib\.go' main.stderr
+# Note that ExportedLibMethod isn't obfuscated.
+! grep 'ExportedLib(Type|Field)|unexportedMainFunc|test/main|main\.go|lib\.go' main.stderr
 
 stdin main.stderr
 garble reverse
 cmp stdout reverse.stdout
 
+! garble build ./build-error
+cp stderr build-error.stderr
+
+stdin build-error.stderr
+garble reverse ./build-error
+cmp stdout build-error-reverse.stdout
+
 [short] stop # no need to verify this with -short
 
 # Ensure that the reversed output matches the non-garbled output.
@@ -93,6 +101,25 @@ func printStackTrace(w io.Writer) error {
 	_, err := w.Write(stack)
 	return err
 }
+-- build-error/error.go --
+package p
+
+import "reflect"
+
+// This program is especially crafted to work with "go build",
+// but fail with "garble build".
+// This is because we attempt to convert from two different struct types,
+// since only the anonymous one has its field name obfuscated.
+// This is useful, because we test that build errors can be reversed,
+// and it also includes a field name.
+
+type UnobfuscatedStruct struct {
+	SomeField int
+}
+
+var _ = reflect.TypeOf(UnobfuscatedStruct{})
+
+var _ = struct{SomeField int}(UnobfuscatedStruct{})
 -- reverse.stdout --
 goroutine 1 [running]:
 runtime/debug.Stack(0x??, 0x??, 0x??)
@@ -107,3 +134,8 @@ main.unexportedMainFunc()
 	test/main/main.go:20 +0x??
 main.main()
 	test/main/main.go:10 +0x??
+-- build-error-reverse.stdout --
+# test/main/build-error
+test/main/build-error/error.go:18: cannot convert UnobfuscatedStruct{} (type UnobfuscatedStruct) to type struct { SomeField int }
+exit status 2
+exit status 2