cmd/cgo: ignore top-level qualifiers in function args/results

The top-level qualifiers are unimportant for our purposes. If a C
function is defined as `const int f(const int i)`, the `const`s are
meaningless to C, and we want to avoid using them in the struct we
create where the `const` has a completely different meaning.

This unwinds https://golang.org/cl/33097 with regard to top-level
qualifiers.

Change-Id: I3d66b0eb43b6d9a586d9cdedfae5a2306b46d96c
Reviewed-on: https://go-review.googlesource.com/33325
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

misc/cgo/test/issue17537.go

@@ -20,7 +20,7 @@ int I17537(S17537 *p);
 #define I17537(p) ((p)->i)
 
 // Calling this function used to fail without the cast.
-int F17537(const char **p) {
+const int F17537(const char **p) {
 	return **p;
 }
 */

src/cmd/cgo/gcc.go

@@ -1476,6 +1476,19 @@ func base(dt dwarf.Type) dwarf.Type {
 	return dt
 }
 
+// unqual strips away qualifiers from a DWARF type.
+// In general we don't care about top-level qualifiers.
+func unqual(dt dwarf.Type) dwarf.Type {
+	for {
+		if d, ok := dt.(*dwarf.QualType); ok {
+			dt = d.Type
+		} else {
+			break
+		}
+	}
+	return dt
+}
+
 // Map from dwarf text names to aliases we use in package "C".
 var dwarfToName = map[string]string{
 	"long int":               "long",
@@ -1930,7 +1943,7 @@ func isStructUnionClass(x ast.Expr) bool {
 // FuncArg returns a Go type with the same memory layout as
 // dtype when used as the type of a C function argument.
 func (c *typeConv) FuncArg(dtype dwarf.Type, pos token.Pos) *Type {
-	t := c.Type(dtype, pos)
+	t := c.Type(unqual(dtype), pos)
 	switch dt := dtype.(type) {
 	case *dwarf.ArrayType:
 		// Arrays are passed implicitly as pointers in C.
@@ -1994,7 +2007,7 @@ func (c *typeConv) FuncType(dtype *dwarf.FuncType, pos token.Pos) *FuncType {
 	if _, ok := dtype.ReturnType.(*dwarf.VoidType); ok {
 		gr = []*ast.Field{{Type: c.goVoid}}
 	} else if dtype.ReturnType != nil {
-		r = c.Type(dtype.ReturnType, pos)
+		r = c.Type(unqual(dtype.ReturnType), pos)
 		gr = []*ast.Field{{Type: r.Go}}
 	}
 	return &FuncType{