support inline comments in asm #include lines

That is, the assembly line

    #include "foo.h" // bar

would make garble run into an error, as we would try to parse
the #include directive before we stripped comments,
effectively trying to unquote the string

    "foo.h" // bar

rather than just the included filename

    "foo.h"

Add test cases for asm but also cgo, while at it.

Fixes #812.

main.go

@@ -688,14 +688,24 @@ func (tf *transformer) transformAsm(args []string) ([]string, error) {
 		for scanner.Scan() {
 			line := scanner.Text()
 
-			// First, handle hash directives without leading whitespaces.
+			// Whole-line comments might be directives, leave them in place.
+			// For example: //go:build race
+			// Any other comment, including inline ones, can be discarded entirely.
+			line, comment, hasComment := strings.Cut(line, "//")
+			if hasComment && line == "" {
+				buf.WriteString("//")
+				buf.WriteString(comment)
+				buf.WriteByte('\n')
+				continue
+			}
 
-			// #include "foo.h"
+			// Preprocessor lines to include another file.
+			// For example: #include "foo.h"
 			if quoted := strings.TrimPrefix(line, "#include"); quoted != line {
 				quoted = strings.TrimSpace(quoted)
 				path, err := strconv.Unquote(quoted)
-				if err != nil {
-					return nil, err
+				if err != nil { // note that strconv.Unquote errors do not include the input string
+					return nil, fmt.Errorf("cannot unquote %q: %v", quoted, err)
 				}
 				newPath := newHeaderPaths[path]
 				switch newPath {
@@ -734,16 +744,8 @@ func (tf *transformer) transformAsm(args []string) ([]string, error) {
 				continue
 			}
 
-			// Leave "//" comments unchanged; they might be directives.
-			line, comment, hasComment := strings.Cut(line, "//")
-
 			// Anything else is regular assembly; replace the names.
 			tf.replaceAsmNames(&buf, []byte(line))
-
-			if hasComment {
-				buf.WriteString("//")
-				buf.WriteString(comment)
-			}
 			buf.WriteByte('\n')
 		}
 		if err := scanner.Err(); err != nil {

testdata/script/asm.txtar

@@ -65,7 +65,7 @@ func main() {
 -- garble_main_amd64.s --
 #include "garble_define_amd64.h"
 
-#include "extra/garble_define2_amd64.h"
+#include "extra/garble_define2_amd64.h" // Inline: many·special∕asm·runes.
 
 // A comment may include many·special∕asm·runes and it's okay.
 //No space: many·special∕asm·runes.

testdata/script/cgo.txtar

@@ -63,7 +63,7 @@ import (
 )
 
 /*
-#include "separate.h"
+#include "separate.h" // inline comment
 
 static int privateAdd(int a, int b) {
 	return a + b;