Make getExceptionMessage work with ASSERTIONS

`getExceptionMessage` didn't work with `-sASSERTIONS`, which is the
default mode for `-O0`, because `getExceptionMessage` assumes the input
`ptr` is a pointer, whereas when `ASSERTIONS` is set, it is an instance
of `CppException`. `in/decrementExceptionRefcount` have the same
problem. Now these methods check whether the pointer is a value or an
instance of `CppException`.

Fixes
emscripten-core#17115 (comment).

Author: aheejin

src/lib/libexceptions.js

@@ -346,13 +346,34 @@ var LibraryExceptions = {
 
 #elif !DISABLE_EXCEPTION_CATCHING
   $incrementExceptionRefcount__deps: ['__cxa_increment_exception_refcount'],
-  $incrementExceptionRefcount: (ptr) => ___cxa_increment_exception_refcount(ptr),
+  $incrementExceptionRefcount: (ptr) => {
+#if EXCEPTION_STACK_TRACES
+    if (ptr instanceof CppException) {
+      ptr = ptr.excPtr;
+    }
+#endif
+    ___cxa_increment_exception_refcount(ptr);
+  },
 
   $decrementExceptionRefcount__deps: ['__cxa_decrement_exception_refcount'],
-  $decrementExceptionRefcount: (ptr) => ___cxa_decrement_exception_refcount(ptr),
+  $decrementExceptionRefcount: (ptr) => {
+#if EXCEPTION_STACK_TRACES
+    if (ptr instanceof CppException) {
+      ptr = ptr.excPtr;
+    }
+#endif
+    ___cxa_decrement_exception_refcount(ptr);
+  },
 
   $getExceptionMessage__deps: ['$getExceptionMessageCommon'],
-  $getExceptionMessage: (ptr) => getExceptionMessageCommon(ptr),
+  $getExceptionMessage: (ptr) => {
+#if EXCEPTION_STACK_TRACES
+    if (ptr instanceof CppException) {
+      ptr = ptr.excPtr;
+    }
+#endif
+    return getExceptionMessageCommon(ptr);
+  },
 
 #endif
 };

test/test_core.py

@@ -1507,7 +1507,6 @@ def test_exceptions_rethrow_missing(self):
 
   @with_all_eh_sjlj
   def test_EXPORT_EXCEPTION_HANDLING_HELPERS(self):
-    self.set_setting('ASSERTIONS', 0)
     self.set_setting('EXPORT_EXCEPTION_HANDLING_HELPERS')
     # FIXME Temporary workaround. See 'FIXME' in the test source code below for
     # details.
@@ -1568,7 +1567,9 @@ class myexception : public exception {
 char const*,
 '''
 
-    self.do_runf('main.cpp', expected)
+    for assertions in (0, 1):
+      self.set_setting('ASSERTIONS', assertions)
+      self.do_runf('main.cpp', expected)
 
   @with_all_eh_sjlj
   def test_bad_typeid(self):