replace project's <LangVersion> based on -langVersion option of "csc.rsp" file

// csc.rsp
-langVersion:10

// *.csproj
...
    <LangVersion>10.0</LangVersion>
...

// csc.rsp
-langVersion:preview

// *.csproj
...
    <LangVersion>11.0</LangVersion>
...