Clang: Fix SOFT ASSERT: "!m_functionName.isEmpty()"

...that ocurred for e.g. "foo (<COMPLETE HERE>".

We did not handle the following cases:
 1) white space after the function name
 2) nothing before parenthesis

Change-Id: If6aedd2cc938df30516e13a860d07d7a509633ae
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>

src/plugins/clangcodemodel/clangcompletioncontextanalyzer.h

@@ -64,7 +64,12 @@ public:
 private:
     ClangCompletionContextAnalyzer();
 
-    struct FunctionInfo { int functionNamePosition; QString functionName; };
+    struct FunctionInfo {
+        bool isValid() const { return functionNamePosition != -1 && !functionName.isEmpty(); }
+
+        int functionNamePosition = -1;
+        QString functionName;
+    };
     FunctionInfo analyzeFunctionCall(int endOfExpression) const;
 
     void setActionAndClangPosition(CompletionAction action, int position);