Auto merge of #16257 - lnicola:rustc-check-cfg, r=Veykril

internal: Teach cargo about `cfg(rust_analyzer)`

r? `@Urgau` is this a good idea?, CC `@Veykril`

crates/proc-macro-srv/proc-macro-test/imp/build.rs

@@ -0,0 +1,5 @@
+//! This teaches cargo about our cfg(rust_analyzer)
+
+fn main() {
+    println!("cargo:rustc-check-cfg=cfg(rust_analyzer)");
+}