Remove a useless assertion in clang-check.

Remove a useless assertion in clang-check.

The assertion prevents it from applying fixes when used along with compilation
databases with relative paths. Added a test that demonstrates the assertion
failure.

An example of the assertion:
input.cpp:11:14: error: expected ';' after top level declarator
typedef int T
             ^
             ;
input.cpp:11:14: note: FIX-IT applied suggested code changes
clang-check: clang/tools/clang-check/ClangCheck.cpp:94: virtual std::string (anonymous namespace)::FixItOptions::RewriteFilename(const std::string &, int &): Assertion `llvm::sys::path::is_absolute(filename) && "clang-fixit expects absolute paths only."' failed.
  #0 llvm::sys::PrintStackTrace(llvm::raw_ostream&) llvm/lib/Support/Unix/Signals.inc:494:13
  #1 llvm::sys::RunSignalHandlers() llvm/lib/Support/Signals.cpp:69:18
  #2 SignalHandler(int) llvm/lib/Support/Unix/Signals.inc:357:1
  #3 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x110c0)
  #4 raise (/lib/x86_64-linux-gnu/libc.so.6+0x32fcf)
  #5 abort (/lib/x86_64-linux-gnu/libc.so.6+0x343fa)
  #6 (/lib/x86_64-linux-gnu/libc.so.6+0x2be37)
  #7 (/lib/x86_64-linux-gnu/libc.so.6+0x2bee2)
  #8 void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag)
  #9 void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char*>(char*, char*, std::__false_type)
 #10 void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*)
 #11 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)
 #12 (anonymous namespace)::FixItOptions::RewriteFilename(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int&) clang/tools/clang-check/ClangCheck.cpp:101:0
 #13 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_data() const
 #14 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_is_local() const
 #15 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_dispose()
 #16 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()
 #17 clang::FixItRewriter::WriteFixedFiles(std::vector<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >*) clang/lib/Frontend/Rewrite/FixItRewriter.cpp:98:0
 #18 std::__shared_ptr<clang::CompilerInvocation, (__gnu_cxx::_Lock_policy)2>::get() const
 #19 std::__shared_ptr_access<clang::CompilerInvocation, (__gnu_cxx::_Lock_policy)2, false, false>::_M_get() const
 #20 std::__shared_ptr_access<clang::CompilerInvocation, (__gnu_cxx::_Lock_policy)2, false, false>::operator->() const
 #21 clang::CompilerInstance::getFrontendOpts() clang/include/clang/Frontend/CompilerInstance.h:290:0
 #22 clang::FrontendAction::EndSourceFile() clang/lib/Frontend/FrontendAction.cpp:966:0
 #23 __gnu_cxx::__normal_iterator<clang::FrontendInputFile*, std::vector<clang::FrontendInputFile, std::allocator<clang::FrontendInputFile> > >::operator++()
 #24 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) clang/lib/Frontend/CompilerInstance.cpp:943:0
 #25 clang::tooling::FrontendActionFactory::runInvocation(std::shared_ptr<clang::CompilerInvocation>, clang::FileManager*, std::shared_ptr<clang::PCHContainerOperations>, clang::DiagnosticConsumer*) clang/lib/Tooling/Tooling.cpp:369:33
 #26 clang::tooling::ToolInvocation::runInvocation(char const*, clang::driver::Compilation*, std::shared_ptr<clang::CompilerInvocation>, std::shared_ptr<clang::PCHContainerOperations>) clang/lib/Tooling/Tooling.cpp:344:18
 #27 clang::tooling::ToolInvocation::run() clang/lib/Tooling/Tooling.cpp:329:10
 #28 clang::tooling::ClangTool::run(clang::tooling::ToolAction*) clang/lib/Tooling/Tooling.cpp:518:11
 #29 main clang/tools/clang-check/ClangCheck.cpp:187:15

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@357915 91177308-0d34-0410-b5e6-96231b3b80d8