clang-check: rename the local FixItAction

Resolve the ambiguity in the FixItAction definition by renaming the type.  With
Xcode 9.2, you would fail to build this with:

  llvm/tools/clang/tools/clang-check/ClangCheck.cpp:183:48: error: reference to 'FixItAction' is ambiguous
      FrontendFactory = newFrontendActionFactory<FixItAction>();
                                                 ^

llvm-svn: 341877