[OpenMP] Introduce new flag to change offloading driver pipeline

This patch introduces the `-fopenmp-new-driver` option which instructs
the compiler to use a new driver scheme for producing offloading code.
In this scheme we create a complete offloading object file and then pass
it as input to the host compilation phase. This will allow us to embed
the object code in the backend phase.

This is the start of a series of commits to rework the OpenMP offloading driver
pipeline. The goal of this is to simplify the steps required for creating an
offloading program. This patch changes the driver's configuration to simply pass
the device file back to the host as an input so it can be embedded as an LLVM IR
global during the backend, then simply passes that object file to the linker.

This driver implementation will currently create the following phases,
```
$ clang input.c -fopenmp -fopenmp-targets=nvptx64 -fopenmp-new-driver -ccc-print-phases
               +- 0: input, "input.c", c, (host-openmp)
            +- 1: preprocessor, {0}, cpp-output, (host-openmp)
         +- 2: compiler, {1}, ir, (host-openmp)
         |        |     +- 3: input, "input.c", c, (device-openmp)
         |        |  +- 4: preprocessor, {3}, cpp-output, (device-openmp)
         |        |- 5: compiler, {4}, ir, (device-openmp)
         |     +- 6: offload, "host-openmp (x86_64-unknown-linux-gnu)" {2}, "device-openmp (nvptx64)" {5}, ir
         |  +- 7: backend, {6}, assembler, (device-openmp)
         |- 8: assembler, {7}, object, (device-openmp)
      +- 9: offload, "host-openmp (x86_64-unknown-linux-gnu)" {2}, "device-openmp (nvptx64)" {8}, ir
   +- 10: backend, {9}, assembler, (host-openmp)
+- 11: assembler, {10}, object, (host-openmp)
12: clang-linker-wrapper, {11}, image, (host-openmp)
```

Which will map to the following bindings

```
# "x86_64-unknown-linux-gnu" - "clang", inputs: ["input.c"], output: "/tmp/input-bae62e.bc"
# "nvptx64" - "clang", inputs: ["input.c", "/tmp/input-bae62e.bc"], output: "/tmp/input-76784e.s"
# "nvptx64" - "NVPTX::Assembler", inputs: ["/tmp/input-76784e.s"], output: "/tmp/input-8f29db.o"
# "x86_64-unknown-linux-gnu" - "clang", inputs: ["/tmp/input-bae62e.bc", "/tmp/input-8f29db.o"], output: "/tmp/input-545450.o"
# "x86_64-unknown-linux-gnu" - "Offload::Linker", inputs: ["/tmp/input-545450.o"], output: "a.out"
```

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D116541