Emit an error if -Z instrument-xray is not supported

This is somewhat important because LLVM enables the pass based on
target architecture, but support by the target OS also matters.

For example, XRay attributes are processed by codegen for macOS
targets, but Apple linker fails to process relocations in XRay
data sections, so the feature as a whole is not supported there
for the time being.

compiler/rustc_session/src/errors.rs

@@ -71,6 +71,12 @@ pub struct ProfileSampleUseFileDoesNotExist<'a> {
 #[diag(session_target_requires_unwind_tables)]
 pub struct TargetRequiresUnwindTables;
 
+#[derive(Diagnostic)]
+#[diag(session_instrumentation_not_supported)]
+pub struct InstrumentationNotSupported {
+    pub us: String,
+}
+
 #[derive(Diagnostic)]
 #[diag(session_sanitizer_not_supported)]
 pub struct SanitizerNotSupported {