Even though 'ClangDiagnosticConsumer' filters out 'clang::diag::err_module_not_found' from being emitted by the Swift compiler, delegating to Swift's module-loading logic for error-handling, we must also ensure that we reset Clang's 'DiagnosticEngine''s error count. Otherwise, if we do not, final stages of IRGen will query Clang's code-gen to finalize its 'Module', which Clang will not do if its internal DiagnosticEngine contains errors. This will cause Swift's IRGen to fail, even though the only error emitted was one Swift intended to suppress.
