ClangCodeModel: Proposal widget displays code snippet where not applicable

Steps to reproduce:

• set QC's codemodel to use ClangCodeModel
• open tests/manual/cplusplus-tools project
• inside main.cpp find the method testCompleteSymbols()
• place cursor onto line

  //    Dummy::s
  

• uncomment the line and trigger auto-completion of this expression

Current result:

• Proposal widget displays "switch" snippet

Note: this wasn't displayed (at least) until QC 3.3