Only the LLVM JIT supports MemorySanitizer instrumentation, so don't use
Subzero for MSan builds.

Bug: b/155148722
Change-Id: I286107b08f36bc1083568353361cfca5947dd4d7
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50308Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>