Fix build on mac.

[zhongweiy]

fix issue #27

[MaskRay]

ccls used to include <experimental/filesystem> which requires -lc++experimental. I have switched to LLVM/Support/FileSystem.h in commit f145c44

I just pushed some commits (transition from libclang indexer to clangIndex !!!) and removed such dependencies

[zhongweiy]

Thanks! It seems we have less dependency now.

However, I've run into another building issue on current master:

/Users/john/projects/ccls/build/clang+llvm-6.0.0-x86_64-apple-darwin/bin/clang++  -DDEFAULT_RESOURCE_DIRECTORY=\"/Users/john/projects/ccls/build/clang+llvm-6.0.0-x86_64-apple-darwin/lib/clang/6.0.0\" -DLOGURU_FILENAME_WIDTH=18 -DLOGURU_THREADNAME_WIDTH=13 -DLOGURU_WITH_STREAMS=1 -I/Users/john/projects/ccls/src -I/Users/john/projects/ccls/third_party -I/Users/john/projects/ccls/third_party/rapidjson/include -isystem /Users/john/projects/ccls/build/clang+llvm-6.0.0-x86_64-apple-darwin/include  -O3 -DNDEBUG   -fno-rtti -Wall -Wno-sign-compare -nostdinc++ -cxx-isystem /Users/john/projects/ccls/build/clang+llvm-6.0.0-x86_64-apple-darwin/include/c++/v1 -std=c++1z -o CMakeFiles/ccls.dir/src/main.cc.o -c /Users/john/projects/ccls/src/main.cc
In file included from /Users/john/projects/ccls/src/main.cc:5:
/Users/john/projects/ccls/src/serializers/json.h:5:10: fatal error: 'rapidjson/document.h' file not found

It would be great if we have some CI checker to ensure the supported platform doesn't break.

[zhongweiy]

The rapidjson/document.h error is caused by forgetting to run git submodule init. Sorry for any inconvenience.