add a test for an IRGen crash

This IRGen crash only shows up with this inliner change: swiftlang#29784 (which is currently reverted)

rdar://problem/59456064

Author: eeckstein

test/IRGen/partial_apply_crash.swift

@@ -0,0 +1,31 @@
+// RUN: %target-swift-frontend -emit-ir -O %s | %FileCheck %s
+
+// This is a compile-only test. It checks that the compiler does not crash for
+// a partial_apply with "unusual" conformances: rdar://problem/59456064
+
+public struct Inner<Element> { }
+
+extension Inner: Equatable where Element: Equatable {
+  public static func == (lhs: Inner<Element>, rhs: Inner<Element>) -> Bool {
+    return false
+  }
+}
+
+public struct Outer<Value> { }
+
+extension Outer: Equatable where Value: Equatable {
+  public static func == (lhs: Outer<Value>, rhs: Outer<Value>) -> Bool {
+    return false
+  }
+}
+
+@inline(never)
+func foo<Element>(_ x: Element, _ equal: (Element, Element) -> Bool) {
+  _ = equal(x, x)
+}
+
+// CHECK: define {{.*}}testit
+public func testit<Element: Equatable>(_ x: Outer<Inner<Element>>) {
+  foo(x, ==)
+}
+