title: CMAKE_CXX_CLANG_TIDY: avoid clang-diagnostic-error interrupting build
body: <p>I am building a C++ project using clang-tidy as linter (cmake -DCMAKE_CXX_CLANG_TIDY=clang-tidy).</p>
  
  <p>After updating my system (Fedora 28->29, cmake 3.11->3.12 I believe), I cannot build any more when clang-tidy reports some clang-diagnostic-error (which I cannot fix right now...). I am pretty sure that clang-diagnostic-error's did not interrupt the build earlier... But I cannot be hundred percent sure.</p>
  
  <p>Edit: The change happened in clang-tidy, now it returns a non-zero exit code when errors are found.</p>
  
  <p>Is it possible to suppress those errors, something like the opposite of "-warnings-as-errors"?</p>