Add save-temps

Author: dianqk

Cargo.toml

@@ -1,3 +1,5 @@
+cargo-features = ["profile-rustflags"]
+
 [workspace]
 resolver = "1"
 members = [
@@ -63,6 +65,11 @@ exclude = [
   "library/stdarch",
 ]
 
+[profile.release.package.rustc_mir_build]
+rustflags = [
+  "-C", "save-temps",
+]
+
 [profile.release.package.compiler_builtins]
 # The compiler-builtins crate cannot reference libcore, and it's own CI will
 # verify that this is the case. This requires, however, that the crate is built