[Driver][Bundler] Improve bundling of object files.

Summary:
Previously, object files were bundled using partial linking. It resulted
in the following structure of the bundled objects:
```
<host_code>
clang-offload-bundle
__CLANG_OFFLOAD_BUNDLE__<target>
<target_code>
```
But when we tried to unbundle object files, it worked correctly only for
the target objects. The host object remains bundled. It produced a lot of
junk sections in the host object files and in some cases may caused
incorrect linking.

Patch improves bundling of the object files. After this patch the
bundled object looks like this:

```
<host_code>
clang-offload-bundle
__CLANG_OFFLOAD_BUNDLE__<target>
<target_code>
__CLANG_OFFLOAD_BUNDLE__<host>
<host_code>
```

With this structure we are able to unbundle the host object files too so
that after unbundling they are the same as were before.
The host section is bundled twice. The bundled section is used to
unbundle the original host section.

Reviewers: yaxunl, tra, jlebar, hfinkel, jdoerfert

Subscribers: caomhin, kkwli0, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65819

llvm-svn: 369019