ClangCodeModel: Switch to LSP-based UI header approach

Generating ui headers in a well-known path and then including that one
in the compilation database does not work in the presence of multiple ui
files with the same name.
As it turns out, we don't have to generate any files at all; instead, we
pass the file contents directly to clangd, which then uses them when
parsing includes of the respective header.
User-visible behavior change apart from the abovementioned bug fix:
Tooltips and "follow symbol" on the include directive now always use the
actual location of the header provided by the build system.

Fixes: QTCREATORBUG-27584
Change-Id: I6b13e12cb3a365199567b0bc824d12b373117697
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/languageclient/languageclientmanager.cpp

@@ -181,7 +181,7 @@ Client *LanguageClientManager::startClient(const BaseSettings *setting,
     return client;
 }
 
-QList<Client *> LanguageClientManager::clients()
+const QList<Client *> LanguageClientManager::clients()
 {
     QTC_ASSERT(managerInstance, return {});
     return managerInstance->m_clients;