[clang] [clang][driver] Don't emit an error on an unrecognized SDK name (PR #181897)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> I think this needs a `FIXME` at minimum.

Sure.

> > arm64-apple-visionos1.0-simulator for XRSimulator1.0.sdk/usr/include/libxml, or at least that's about as close to valid as we'll be able to figure out since we don't know if there's something special about libxml that might require a different triple.
> 
> My point is that given this target triple and sysroot, the real sysroot should be `XRSimulator1.0.sdk` where there is infact a `SDKSettings` file. This is pointing at user error, and if it's not user error because they know what they are doing, there should be a way for the user to opt out of the diagnostic, like with -Wno-incompatible-sysroot. While there may be legit reasons to handle it, it can also very well be a mistake, and my feeling is that should be the average case the compiler expects.

I don't think -Wincompatible-sysroot is the right diagnostic, probably a new -Winvalid-sysroot would be better. i.e. it's not that -isysroot and -target are incompatible, it's that sysroot doesn't have an SDKSettings file or otherwise look like a sysroot.

https://github.com/llvm/llvm-project/pull/181897