[llvm-profgen] Support to load debug info from a second binary

For reducing binary size purpose, the binary's debug info and executable segment can be separated(like using objcopy --only-keep-debug). Here add support in llvm-profgen to use two binaries as input. The original one is executable binary and added for debug info only binary. Adding a flag `--debug-binary=file-path`, with this, the binary will load debug info from debug binary.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D115948