Build with LLVM/Clang 6.0 instead of latest available (#1655852)
    
    It seems that the only difference between the official binaries
    and binaries from Fedora 29 repositories is the version of used
    LLVM/Clang. Qt Creator in F29 uses LLVM/Clang 7.0, while official
    build of Qt Creator uses LLVM/Clang 6.0.
    
    And while it looks like Qt Creator works fine with both versions,
    it fails to parse most of the Linux kernel source code when compiled
    with LLVM 7.0.