Reduce Runtime's dependence to LLVM

[chentong319]

1. Replace the llvm::system with std::filesystem
2. replace llvm::StringRef.compareCaseInsentive with implementation directly with std::string.
3. Keep the llvm StandLibrary only for windows.

As a result, the code previous under PYRUNTIME_LIGHT for src/Runtime becomes the default code now.

Now the only source code difference is the float16 support in src/Runtime/python. There are too much code to copy from llvm. Low priority.

[AlexandreEichenberger]

Minor: please add // Please no references to LLVM in the else path just as a reminder that we don't reintroduce dependences on LLVM.

[chentong319]

Those llvm includes are removed because they are replaced with STLs of C++.