Fix loplugin:useuniqueptr for libc++ (macOS)

...after 05a337e2 "loplugin:useuniqueptr look
for deleting in loops with iterators", where it didn't emit the warning for
Foo24 in compilerplugins/clang/test/useuniqueptr.cxx during
CompilerTest_compilerplugins_clang, because in the initialization of

  HTMLAttrs::const_iterator it = m_aSetAttrTab.begin();

the HTMLAttrs::const_iterator CXXConstructExpr happens to have a second,
defaulted argument.

Change-Id: I882a6dfb5cab1b147f790072f2545b13172c0f9a
Reviewed-on: https://gerrit.libreoffice.org/61530
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

compilerplugins/clang/useuniqueptr.cxx

@@ -497,8 +497,11 @@ void UseUniquePtr::CheckLoopDelete(const FunctionDecl* functionDecl, const CXXDe
                     {
                         init = init->IgnoreImplicit();
                         if (auto x = dyn_cast<CXXConstructExpr>(init))
-                            if (x->getNumArgs() == 1)
+                            if (x->getNumArgs() == 1
+                                || (x->getNumArgs() >= 2 && isa<CXXDefaultArgExpr>(x->getArg(1))))
+                            {
                                 init = x->getArg(0)->IgnoreImplicit();
+                            }
                         if (auto x = dyn_cast<CXXMemberCallExpr>(init))
                             init = x->getImplicitObjectArgument();
                         if ((memberExpr = dyn_cast<MemberExpr>(init)))