cpp_import: Let Jakt know if imported methods are virtual

This allows Jakt classes to override methods defined in C++ classes.

Author: LucasChollet

selfhost/cpp_import/clang.jakt

@@ -1741,6 +1741,8 @@ class CppImportProcessor {
             is_method and clang_CXXMethod_isStatic(cursor)
         ) or not is_method
 
+        let is_virtual = clang_CXXMethod_isVirtual(cursor)
+
         if not this_type.has_value() and (kind is Constructor or kind is Destructor) {
             if .debug_print { eprintln("[ICE] Constructor or destructor ({}) without a this type", name_from(cursor)) }
             return
@@ -1907,7 +1909,7 @@ class CppImportProcessor {
             is_instantiated: true
             parsed_function: None
             is_comptime: false
-            is_virtual: false
+            is_virtual: is_virtual
             is_override: false
             is_unsafe: false
             has_varargs

tests/typechecker/polymorphism_override_cpp_method.h

@@ -0,0 +1,4 @@
+class Parent {
+protected:
+    virtual void f() { AK::outln("From the parent"); };
+};

tests/typechecker/polymorphism_override_cpp_method.jakt

@@ -0,0 +1,14 @@
+/// Expect:
+/// - output: "From the child\n"
+/// - cppincludes: "."
+
+import extern "polymorphism_override_cpp_method.h"
+
+class Child : Parent {
+    public override fn f(mut this) { print("From the child\n"); }
+}
+
+fn main() {
+    mut c = Child()
+    c.f()
+}