Add splat mangling bug tests (flaky?)

Author: teor2345

tests/ui/splat/splat-mangling.rs

@@ -0,0 +1,18 @@
+//! Test that splat is used in symbol mangling.
+//@ build-fail
+//@ incremental
+//@ compile-flags: -C opt-level=0
+
+#![allow(incomplete_features)]
+#![feature(splat)]
+
+fn main() {
+    // Bug #158603 regression test variants
+    #[rustfmt::skip]
+    let _x: fn(#[splat] (i32,)) = None.unwrap();
+
+    //@ regex-error-pattern: symbol `.*Option.*unwrap.*splat_mangling` is already defined
+    //~? ERROR: is already defined
+    let x: fn((i32,)) = None.unwrap();
+    x((1,));
+}

tests/ui/splat/splat-mangling.stderr

@@ -0,0 +1,4 @@
+error: symbol `_RNvMNtCsCRATE_HASH_4core6optionINtB2_6OptionFTlEEuE6unwrapCsCRATE_HASH_14splat_mangling` is already defined
+
+error: aborting due to 1 previous error
+