cgo: implement rudimentary C array decaying

This is just a first step. It's not complete, but it gets some real
world C code to parse.

This signature, from the ESP-IDF:

    esp_err_t esp_wifi_get_mac(wifi_interface_t ifx, uint8_t mac[6]);

Was previously converted to something like this (pseudocode):

    C.esp_err_t esp_wifi_get_mac(ifx C.wifi_interface_t, mac [6]uint8)

But this is not correct. C array parameters will decay. The array is
passed by reference instead of by value. Instead, this would be the
correct signature:

    C.esp_err_t esp_wifi_get_mac(ifx C.wifi_interface_t, mac *uint8)

So that it can be called like this (using CGo):

    var mac [6]byte
    errCode := C.esp_wifi_get_mac(C.ESP_IF_WIFI_AP, &mac[0])

This stores the result in the 6-element array mac.

testdata/cgo/main.c

@@ -61,3 +61,7 @@ void unionSetData(short f0, short f1, short f2) {
 	globalUnion.data[1] = 8;
 	globalUnion.data[2] = 1;
 }
+
+void arraydecay(int buf1[5], int buf2[3][8], int buf3[4][7][2]) {
+	// Do nothing.
+}

testdata/cgo/main.go

@@ -128,6 +128,12 @@ func main() {
 	// Check whether CFLAGS are correctly passed on to compiled C files.
 	println("CFLAGS value:", C.cflagsConstant)
 
+	// Check array-to-pointer decaying. This signature:
+	//   void arraydecay(int buf1[5], int buf2[3][8], int buf3[4][7][2]);
+	// decays to:
+	//   void arraydecay(int *buf1, int *buf2[8], int *buf3[7][2]);
+	C.arraydecay((*C.int)(nil), (*[8]C.int)(nil), (*[7][2]C.int)(nil))
+
 	// libc: test whether C functions work at all.
 	buf1 := []byte("foobar\x00")
 	buf2 := make([]byte, len(buf1))

testdata/cgo/main.h

@@ -144,3 +144,7 @@ extern int cflagsConstant;
 // test duplicate definitions
 int add(int a, int b);
 extern int global;
+
+// Test array decaying into a pointer.
+typedef int arraydecay_buf3[4][7][2];
+void arraydecay(int buf1[5], int buf2[3][8], arraydecay_buf3 buf3);