Clang: Remember selected function signature hint

...when typing more arguments:

    struct Foo {};
    void f(int, int);
    void f(Foo, Foo);
    void f(char, char);

    void c()
    {
        f( // 1. Trigger completion with Ctrl+Space
           // 2. Chose item "f(Foo, Foo)"
           // 3. Type: Foo(),
           // OK, signature hint "f(Foo, Foo)" is displayed again
    }

FunctionHintProposalWidget and IFunctionHintProposalModel are
instantiated for each calculation, so remember the selected hint in the
CodeAssist. Keep the latest 20 entries.

Task-number: QTCREATORBUG-11688
Change-Id: I579fc6d8a35dd8fa398e4b3170ddc05a85252d1a
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/clangcodemodel/clangfunctionhintmodel.cpp

@@ -61,6 +61,15 @@ QString ClangFunctionHintModel::text(int index) const
     return signatureWithEmphasizedCurrentParameter;
 }
 
+QString ClangFunctionHintModel::id(int index) const
+{
+    QString chunks;
+    for (const ClangBackEnd::CodeCompletionChunk &chunk : m_functionSymbols.at(index).chunks())
+        chunks += chunk.text();
+
+    return chunks;
+}
+
 int ClangFunctionHintModel::activeArgument(const QString &prefix) const
 {
     int activeArgumentNumber = 0;

src/plugins/clangcodemodel/clangfunctionhintmodel.h

@@ -40,6 +40,7 @@ public:
     void reset() override;
     int size() const override;
     QString text(int index) const override;
+    QString id(int index) const override;
     int activeArgument(const QString &prefix) const override;
 
 private: