Public Clang calls itself "Clang"; Apple Clang calls itself "Apple LLVM".
The test doesn't actually care what the name is; it's just trying to make
sure that the importer is failing for the reason we expect, rather than
something unrelated.
rdar://problem/23506925
This is most important for -Xcc options, which shouldn't be dependent on
the underlying Clang version, but it also affects things like target features
that Clang knows how to compute correctly and Swift, well, doesn't.
rdar://problem/17687572
Swift SVN r22448
