By now the context's SourceManager is now initialized everywhere that
ClangTidyCheck::registerMatcher() is called, so the call from run() seems
entirely redundant, and indeed all the tests pass.
This solves a problem with embedding clang-tidy: if using a DiagnosticsEngine
which already has file state, re-setting its SourceManager (to the same value)
causes an assertion.
(There are other ways to solve this problem, but this is the simplest).