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() {}

tests/codegen/instrument-xray/options-combine.rs

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+// Checks that `-Z instrument-xray` options can be specified multiple times.
+//
+// compile-flags: -Z instrument-xray=skip-exit
+// compile-flags: -Z instrument-xray=instruction-threshold=123
+// compile-flags: -Z instrument-xray=instruction-threshold=456
+
+#![crate_type = "lib"]
+
+// CHECK:      attributes #{{.*}} "xray-instruction-threshold"="456" "xray-skip-exit"
+// CHECK-NOT:  attributes #{{.*}} "xray-instruction-threshold"="123"
+pub fn function() {}

tests/codegen/instrument-xray/options-override.rs

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+// Checks that the last `-Z instrument-xray` option wins.
+//
+// compile-flags: -Z instrument-xray=always
+// compile-flags: -Z instrument-xray=never
+
+#![crate_type = "lib"]
+
+// CHECK:      attributes #{{.*}} "function-instrument"="xray-never"
+// CHECK-NOT:  attributes #{{.*}} "function-instrument"="xray-always"
+pub fn function() {}