Clang: Fix delayed reparse of dirty and visible but not current documents

1. Open document foo.h
2. Create a split and open foo.cpp (#including "foo.h")
3. Edit foo.h (e.g. by introducing a syntax error, so that foo.cpp will
   indicate header errors in the toolbar or as info bar)
=> Actual: foo.cpp will be reparsed immediately.
 Expected: foo.cpp should be reparsed after a delay.

This saves resources (cpu time) and minimizes poping up of the header
info bar while editing header files in splits.

Regression introduced by

    commit 380d756a03
    Clang: Hook up supportive translation unit on first edit

Change-Id: Ib5fd90e49415dfc3aefacab7cd627b0e1937f5fc
Reviewed-by: David Schulz <david.schulz@qt.io>

src/tools/clangbackend/ipcsource/clangdocuments.cpp

@@ -33,6 +33,8 @@
 #include <skippedsourceranges.h>
 #include <unsavedfiles.h>
 
+#include <utils/algorithm.h>
+
 #include <QDebug>
 
 #include <algorithm>
@@ -143,6 +145,20 @@ const std::vector<Document> &Documents::documents() const
     return documents_;
 }
 
+const std::vector<Document> Documents::filtered(const IsMatchingDocument &isMatchingDocument) const
+{
+    return Utils::filtered(documents_, isMatchingDocument);
+}
+
+std::vector<Document> Documents::dirtyAndVisibleButNotCurrentDocuments() const
+{
+    return filtered([](const Document &document) {
+        return document.isNeedingReparse()
+            && document.isVisibleInEditor()
+            && !document.isUsedByCurrentEditor();
+    });
+}
+
 UnsavedFiles Documents::unsavedFiles() const
 {
     return unsavedFiles_;