[clang-cl] Explicitly set object format to COFF in CL mode

Summary:
Currently object format is taken from the default target triple. For toolchains with a non-COFF default target this may result in an object format inappropriate for pc-windows and lead to compilation issues. 

For example, the default triple `aarch64-linux-elf` may produce something like `aarch64-pc-windows-msvc19.0.24215-elf` in CL mode. Clang creates `MicrosoftARM64TargetInfo` for such triple with data layout `e-m:w-p:64:64-i32:32-i64:64-i128:128-n32:64-S128`. On the other hand, the AArch64 backend in `computeDataLayout` detects a non-COFF target and selects `e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128` as data layout for little endian. Different layouts used by clang and the backend cause an error:
```
error: backend data layout 'e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128'
 does not match expected target description 'e-m:w-p:64:64-i32:32-i64:64-i128:128-n32:64-S128'
```
This can be observed on the clang's Driver/cl-pch.c test with AArch64 as a default target.

This patch enforces COFF in CL mode.

Reviewers: hans

Reviewed By: hans

Subscribers: cfe-commits, aemerson, asl, kristof.beyls

Differential Revision: https://reviews.llvm.org/D37336

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312275 91177308-0d34-0410-b5e6-96231b3b80d8