[LTO API] split lto_codegen_compile to lto_codegen_optimize and

lto_codegen_compile_optimized. Also add lto_api_version.

Before this commit, we can only dump the optimized bitcode after running
lto_codegen_compile, but it includes some impacts of running codegen passes,
one example is StackProtector pass. We will get assertion failure when running
llc on the optimized bitcode, because StackProtector is effectively run twice.

After splitting lto_codegen_compile, the linker can choose to dump the bitcode
before running lto_codegen_compile_optimized.

lto_api_version is added so ld64 can check for runtime-availability of the new
API.

rdar://19565500

llvm-svn: 228000