Utils: Replace FilePath::onDevice() by new FilePath::withMappedPath()

Basically a.onDevice(b) == b.withNewMappedPath(a), matching the order
of b.withNewPath(a).

Whether the (curretly docker-specific) path mapping is useful /there/, and
whether some of the calls are needed at all is dubious. I added some
FIXME and changed a few cases directly.

Change-Id: I7514736ce922f632f1f737bc496f6783389a42b6
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/languageclient/client.cpp

@@ -2114,7 +2114,7 @@ FilePath Client::serverUriToHostPath(const LanguageServerProtocol::DocumentUri &
 DocumentUri Client::hostPathToServerUri(const Utils::FilePath &path) const
 {
     return DocumentUri::fromFilePath(path, [&](const Utils::FilePath &clientPath) {
-        return clientPath.onDevice(d->m_serverDeviceTemplate);
+        return d->m_serverDeviceTemplate.withNewPath(clientPath.path());
     });
 }