[clang-offload-bundler] Library-ize ClangOffloadBundler

Lifting the core functionalities of the clang-offload-bundler into a
user-facing library/API. This will allow online and JIT compilers to
bundle and unbundle files without spawning a new process.

This patch lifts the classes and functions used to implement
the clang-offload-bundler into a separate OffloadBundler.cpp,
and defines three top-level API functions in OfflaodBundler.h.
        BundleFiles()
        UnbundleFiles()
        UnbundleArchives()

This patch also introduces a Config class that locally stores the
previously global cl::opt options and arrays to allow users to call
the APIs in a multi-threaded context, and introduces an
OffloadBundler class to encapsulate the top-level API functions.

We also  lift the BundlerExecutable variable, which is specific
to the clang-offload-bundler tool, from the API, and replace
its use with an ObjcopyPath variable. This variable must be set
in order to internally call llvm-objcopy.

Finally, we move the API files from
clang/tools/clang-offload-bundler into clang/lib/Driver and
clang/include/clang/Driver.

Differential Revision: https://reviews.llvm.org/D129873