This commit adds -lto flag for frontend to enable LTO at LLVM level.
When -lto=llvm given, compiler emits LLVM bitcode file instead of object
file and adds index summary for LTO.
In addition for ELF format, emit llvm.dependent-libraries section to
embed auto linking information
This is important, for example, for linking correctly to the C++
standard library on Windows, which uses `#pragma comment(lib, ...)` in
its headers to specify the correct library to link against.
