ClangCodeModel: Use project-specific clangd client, if possible

If we open a file that does not belong to a currently open project, there
is a good chance that the current project's parse context is a better fit
for the file than the fallback client's.

Fixes: QTCREATORBUG-26697
Change-Id: I6c17e275b047602c51364f3203b3f0a3e74a49fc
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/test/clangdtests.cpp

@@ -2036,7 +2036,7 @@ void ClangdTestExternalChanges::test()
     QVERIFY(waitForSignalOrTimeout(ClangModelManagerSupport::instance(),
                                    &ClangModelManagerSupport::createdClient, timeOutInMs()));
     ClangdClient * const newClient = ClangModelManagerSupport::instance()
-            ->clientForFile(filePath("main.cpp"));
+            ->clientForProject(project());
     QVERIFY(newClient);
     QVERIFY(newClient != oldClient);
     newClient->enableTesting();