ClangToolsUtils: Add isVFSOverlaySupported()

Remove ClangToolRunner::supportsVFSOverlay().
Move check for modified document and vfsOverlay into
runner creator method, as it's called in the same cycle just
before the runner is about to be started.

Change-Id: I7a5df71bfa73c350862e4c7f9eae49773b6206b7
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangtools/clangtoolsutils.cpp

@@ -200,6 +200,19 @@ FilePath toolFallbackExecutable(ClangToolType tool)
     return findValidExecutable({toolShippedExecutable(tool), fallback});
 }
 
+bool isVFSOverlaySupported(const FilePath &executable)
+{
+    static QMap<FilePath, bool> vfsCapabilities;
+    auto it = vfsCapabilities.find(executable);
+    if (it == vfsCapabilities.end()) {
+        QtcProcess p;
+        p.setCommand({executable, {"--help"}});
+        p.runBlocking();
+        it = vfsCapabilities.insert(executable, p.allOutput().contains("vfsoverlay"));
+    }
+    return it.value();
+}
+
 static void addBuiltinConfigs(ClangDiagnosticConfigsModel &model)
 {
     model.appendOrUpdate(builtinConfig());