Revert r222363: "[clang-tidy] Count errors in ClangTidyDiagnosticConsumer"

This was causing clang-tidy invocations that would previously pass despite
source errors to fail.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@222390 91177308-0d34-0410-b5e6-96231b3b80d8