[Runtime] Opt-in for runtime conformance collision warnings.

Make the new runtime conformance collision warnings opt-it, via the
environment variable SWIFT_ENABLE_CONFLICTING_CONFORMANCES_CHECK.

Author: DougGregor

stdlib/public/runtime/ProtocolConformance.cpp

@@ -583,6 +583,11 @@ namespace {
   };
 }
 
+static bool shouldWarnAboutConflictingConformances() {
+  return SWIFT_LAZY_CONSTANT(
+           (bool)getenv("SWIFT_ENABLE_CONFLICTING_CONFORMANCES_CHECK"));
+}
+
 const ProtocolConformanceDescriptor *
 swift::_conformsToSwiftProtocol(const Metadata * const type,
                                 const ProtocolDescriptor *protocol,
@@ -733,7 +738,7 @@ swift::_conformsToSwiftProtocol(const Metadata * const type,
       // If we have only retroactive conformances, complain about the
       // conflict.
       // FIXME: Only do this once per type descriptor/protocol combination.
-      if (!foundNonRetroactive) {
+      if (!foundNonRetroactive && shouldWarnAboutConflictingConformances()) {
         // Form the list of modules in which we found retroactive conformances.
         std::string moduleNamesStr;
         const char *firstModuleName = nullptr;

test/Runtime/multiple_conformances.swift

@@ -4,7 +4,7 @@
 // RUN: %target-build-swift -parse-as-library -force-single-frontend-invocation -I %t %S/Inputs/RetroactiveB.swift -emit-module-path %t/RetroactiveB.swiftmodule -emit-object -o %t/RetroactiveB.o
 // RUN: %target-build-swift -I %t %s %t/RetroactiveCommon.o %t/RetroactiveA.o %t/RetroactiveB.o -module-name main -o %t/a.out 
 // RUN: %target-codesign %t/a.out
-// RUN: %target-run %t/a.out > %t.log 2>&1
+// RUN: env SWIFT_ENABLE_CONFLICTING_CONFORMANCES_CHECK=1 %target-run %t/a.out > %t.log 2>&1
 // RUN: %FileCheck %s < %t.log
 
 // REQUIRES: executable_test