Diagnose the combination of -Ounchecked with strict memory safety

Aligns with the current proposal and fixes rdar://127128995

Author: DougGregor

include/swift/AST/DiagnosticsFrontend.def

@@ -589,5 +589,9 @@ ERROR(experimental_not_supported_in_production,none,
       "experimental feature '%0' cannot be enabled in production compiler",
       (StringRef))
 
+GROUPED_WARNING(Ounchecked_with_strict_safety,Unsafe,none,
+      "'-Ounchecked' is not memory-safe and should not be combined with "
+      "strict memory safety checking", ())
+
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

lib/Frontend/CompilerInvocation.cpp

@@ -3834,6 +3834,13 @@ bool CompilerInvocation::parseArgs(
     }
   }
 
+  if (LangOpts.hasFeature(Feature::WarnUnsafe)) {
+    if (SILOpts.RemoveRuntimeAsserts ||
+        SILOpts.AssertConfig == SILOptions::Unchecked) {
+      Diags.diagnose(SourceLoc(), diag::Ounchecked_with_strict_safety);
+    }
+  }
+
   SILOpts.UseAggressiveReg2MemForCodeSize =
     ParsedArgs.hasFlag(OPT_enable_aggressive_reg2mem,
                        OPT_disable_aggressive_reg2mem,

test/Unsafe/unsafe_Ounchecked.swift

@@ -0,0 +1,5 @@
+// RUN: %target-swift-frontend -typecheck -enable-experimental-feature WarnUnsafe -Ounchecked %s 2>&1 | %FileCheck %s
+
+// REQUIRES: swift_feature_WarnUnsafe
+
+// CHECK: warning: '-Ounchecked' is not memory-safe