[Driver] Add -mllvm= as an alias for -mllvm

[MaskRay]

Summary

Similar to D131455 (-Xclang=). As well as making it convenient for some build
systems, this allows -Xarch_device '-mllvm=--inline-threshold=100' (and
-Xarch_host; so we don't need to allow space separators which are uncommon in
driver code).

Bear in mind that -mllvm options are unstable and should be avoided if possible.

[MaskRay]

fix test

[jhuber6]

This looks good. I've had similar problems when trying to pass things with arguments via the -Xarch or -Xopenmp-target= options.

[yaxunl]

LGTM. Thanks.

[Meinersbur]

This patch broke the Flang buildbot, e.g. https://lab.llvm.org/buildbot/#/builders/172/builds/23305

The tests
https://github.com/llvm/llvm-project/blob/main/flang/test/Driver/driver-help.f90
https://github.com/llvm/llvm-project/blob/main/flang/test/Driver/driver-help-hidden.f90

They check the entire output of -help and need to be updated for new options.

[MaskRay]

In D143325#4105356, @Meinersbur wrote:

This patch broke the Flang buildbot, e.g. https://lab.llvm.org/buildbot/#/builders/172/builds/23305

The tests
https://github.com/llvm/llvm-project/blob/main/flang/test/Driver/driver-help.f90
https://github.com/llvm/llvm-project/blob/main/flang/test/Driver/driver-help-hidden.f90

They check the entire output of -help and need to be updated for new options.

Thanks for the report. Already fixed by https://reviews.llvm.org/rG6a8a423c