[clang] Preliminary fat-lto-object support

[clang] Preliminary fat-lto-object support

Fat LTO objects contain both LTO compatible IR, as well as generated
object code. This allows users to defer the choice of whether to use LTO
or not to link-time. This is a feature available in GCC for some time,
and makes the existing -ffat-lto-objects flag functional in the same
way as GCC's.

This patch adds support for that flag in the driver, as well as setting the
necessary codegen options for the backend. Largely, this means we select
the newly added pass pipeline for generating fat objects.

Users are expected to pass -ffat-lto-objects to clang in addition to one
of the -flto variants. Without the -flto flag, -ffat-lto-objects has no
effect.

// Compile and link. Use the object code from the fat object w/o LTO.
clang -fno-lto -ffat-lto-objects -fuse-ld=lld foo.c

// Compile and link. Select full LTO at link time.
clang -flto -ffat-lto-objects -fuse-ld=lld foo.c

// Compile and link. Select ThinLTO at link time.
clang -flto=thin -ffat-lto-objects -fuse-ld=lld foo.c

// Compile and link. Use ThinLTO  with the UnifiedLTO pipeline.
clang -flto=thin -ffat-lto-objects -funified-lto -fuse-ld=lld foo.c

// Compile and link. Use full LTO  with the UnifiedLTO pipeline.
clang -flto -ffat-lto-objects -funified-lto -fuse-ld=lld foo.c

// Link separately, using ThinLTO.
clang -c -flto=thin -ffat-lto-objects foo.c
clang -flto=thin -fuse-ld=lld foo.o -ffat-lto-objects  # pass --lto=thin --fat-lto-objects to ld.lld

// Link separately, using full LTO.
clang -c -flto -ffat-lto-objects foo.c
clang -flto -fuse-ld=lld foo.o  # pass --lto=full --fat-lto-objects to ld.lld

Original RFC: https://discourse.llvm.org/t/rfc-ffat-lto-objects-support/63977

Depends on D146776

Reviewed By: tejohnson, MaskRay

Differential Revision: https://reviews.llvm.org/D146777