Add test case for disabling lexical lifetimes.

Make sure the option for restoring pre-5.7 lifetime behavior works in
the 5.7 release.

Author: atrick

test/SILOptimizer/lexical_lifetime_elim.swift

@@ -0,0 +1,39 @@
+// RUN: %target-swift-frontend -emit-sil -O -parse-as-library -enable-copy-propagation=false -Xllvm -sil-print-all -module-name=main %s 2>&1 | %FileCheck %s
+// RUN: %target-swift-frontend -emit-sil -O -parse-as-library -enable-lexical-lifetimes=false -Xllvm -sil-print-all -module-name=main %s 2>&1 | %FileCheck %s
+
+@inline(never)
+func takeGuaranteed(_ a: AnyObject) -> AnyObject {
+  return a
+}
+
+// CHECK-LABEL: // testLexical(a:)
+// CHECK: [[B:%.*]] = begin_borrow [lexical] %0
+// CHECK: apply %{{.*}}([[B]])
+// CHECK: apply
+// CHECK: end_borrow [[B]]
+// CHECK-LABEL: } // end sil function
+
+// LexicalLifetimeEliminator must strip the [lexical] flag
+// before the first round of SemanticARCOpts.
+
+// CHECK-NOT: *** SIL function after {{.*}} (semantic-arc-opts)
+
+// CHECK-LABEL: *** SIL function after {{.*}} (sil-lexical-lifetime-eliminator)
+// CHECK-LABEL: // testLexical(a:)
+// CHECK: [[B:%.*]] = begin_borrow %0
+// CHECK: apply %{{.*}}([[B]])
+// CHECK: apply
+// CHECK: end_borrow [[B]]
+// CHECK-LABEL: } // end sil function
+
+// The first round of SemanticARCOpts must eliminate the borrow scope
+// that was only needed for a lexical lifetime.
+
+// CHECK-LABEL: *** SIL function after {{.*}} (semantic-arc-opts)
+// CHECK-LABEL: // testLexical(a:)
+// CHECK: apply %{{.*}}(%0)
+// CHECK-LABEL: } // end sil function
+public func testLexical(a: __owned AnyObject) -> AnyObject {
+  // Without lexical lifetimes, the lifetime of 'a' ends in between the two calls:
+  return takeGuaranteed(takeGuaranteed(a))
+}