Clang: Set limit for sorting completion items

Having *some* limit is reasonable since sorting happens in the UI thread
and given enough includes/symbols it will freeze sooner or later.

The strategy is now: Stop sorting at all if there are more than 30.000
completion items to process.

The limit is a guess made on the following measurements. The goal is to
keep below 100ms.

 test | items | before | now
 -----+-----------------------
 Cpp1 | 6394  | 172ms  | 15ms |
 Win1 | 44739 | 125ms  | 56ms |
 Qtc1 | 9648, |  16ms  | 15ms |
 Qtc2 | 10210 |  20ms  | 16ms |
 Qtc3 | 51044 | 141ms  | 63ms |
 Qt1  | 62953 | 172ms  | 78ms |

Windows.h alone pulls in about 44.000 completion items.

Used test procedure
-------------------
For each test below, do:

 1. Start Qt Creator in release mode
 2. Open/create the project with a MSVC2015 Kit
 3. Open the mentioned file
 4. Strg+Space in the mentioned function

Measured with a timer in IpcReceiver::codeCompleted.

Test projects:
 Cpp1: stdc++11-includes.pro, main.cpp, main()
 Win1: Create project from wizard, *.cpp file including windows.h, completion
       in some added function
 Qtc1: qtcreator.pro, texteditor.cpp, BaseTextEditor::duplicate()
 Qtc2: qtcreator.pro, fakevimhandler.cpp, FakeVimHandler::jumpToLocalMark
 Qtc3: qtcreator.pro, botan.cpp, version_string()
 Qt1:  qt-essential-includes.pro, main.cpp, main()

Change-Id: I6fbd65d14f6086f289be7dd6c24385996e4bde83
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/clangcodemodel/clangassistproposalmodel.cpp

@@ -34,6 +34,8 @@
 namespace ClangCodeModel {
 namespace Internal {
 
+const int SORT_LIMIT = 30000;
+
 ClangAssistProposalModel::ClangAssistProposalModel(
         ClangBackEnd::CompletionCorrection neededCorrection)
     : m_neededCorrection(neededCorrection)
@@ -47,7 +49,7 @@ bool ClangAssistProposalModel::containsDuplicates() const
 
 bool ClangAssistProposalModel::isSortable(const QString &/*prefix*/) const
 {
-    return true;
+    return m_currentItems.size() <= SORT_LIMIT;
 }
 
 void ClangAssistProposalModel::sort(const QString &/*prefix*/)