[clang-tidy] run() doesn't update the SourceManager.

Summary:
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).

Reviewers: hokein, alexfh

Subscribers: xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D54061

llvm-svn: 346219