[clang] [clang][Driver] Support -fms-volatile as equivalent to /volatile:ms (PR #74790)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I got a request for this feature from a user that ran into issues porting code from cl to clang for a baremetal target.  I expect scenarios like that to continue to be relevant for a while.

I don't expect anyone would want to use this for new code because current practice has strongly shifted towards using atomic types for synchronization since C++11.

Like the commit message notes, this isn't changing any defaults; it's just giving users an additional tool for porting code.  It's something we've already provided for many years; it just wasn't exposed outside cl-mode.  And it's a pretty straightforward extension to support (it just slightly modifies IR generation), so I don't think there's much long-term support burden.

https://github.com/llvm/llvm-project/pull/74790