Codegen tests for -Z instrument-xray

Let's add at least some tests to verify that this option is accepted
and produces expected LLVM attributes. More tests can be added later
with attribute support.

tests/codegen/instrument-xray/basic.rs

@@ -0,0 +1,8 @@
+// Checks that `-Z instrument-xray` produces expected instrumentation.
+//
+// compile-flags: -Z instrument-xray=always
+
+#![crate_type = "lib"]
+
+// CHECK: attributes #{{.*}} "function-instrument"="xray-always"
+pub fn function() {}