Utility methods for artificial debug types in the generated DWARF (#1482)

* add operator*
* add operator->
* add ptr() unwrap method
* comments/refactor
* macro_rules
* external symbols workaround

tests/debug/testsuite/reverse-str.c

@@ -0,0 +1,21 @@
+// Compile with:
+//   clang --target=wasm32 reverse-str.c -o reverse-str.wasm -g \
+//     -O0 -nostdlib -fdebug-prefix-map=$PWD=.
+#include <stdlib.h>
+
+void reverse(char *s, size_t len)
+{
+    if (!len) return;
+    size_t i = 0, j = len - 1;
+    while (i < j) {
+        char t = s[i];
+        s[i++] = s[j];
+        s[j--] = t;
+    }
+}
+
+void _start()
+{
+    char hello[] = "Hello, world.";
+    reverse(hello, 13);
+}