[clang-tidy] warnings-as-error no longer exits with ErrorCount

When using `-warnings-as-errors`, If there are any warnings promoted to errors, clang-tidy exits with the number of warnings. This really isn't needed and can cause issues when the number of warnings doesn't fit into 8 bits as POSIX terminals aren't designed to handle more than that.
This addresses https://bugs.llvm.org/show_bug.cgi?id=46305.

Bug originally added in D15528

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D81953