IRGen: apply IR linkage to trivial property descriptors

This ensures that the trivial property descriptors is externally
accessible on targets like Windows which does not export symbols by
default.  Thanks to Joe Groff for the pointer about the global aliases
for trivial property descriptors!

lib/IRGen/GenKeyPath.cpp

@@ -1274,9 +1274,13 @@ void IRGenModule::emitSILProperty(SILProperty *prop) {
     } else {
       auto entity = LinkEntity::forPropertyDescriptor(prop->getDecl());
       auto linkInfo = LinkInfo::get(*this, entity, ForDefinition);
-      llvm::GlobalAlias::create(linkInfo.getLinkage(),
+      auto *GA = llvm::GlobalAlias::create(linkInfo.getLinkage(),
                                            linkInfo.getName(),
                                            TheTrivialPropertyDescriptor);
+      ApplyIRLinkage({linkInfo.getLinkage(),
+                      linkInfo.getVisibility(),
+                      llvm::GlobalValue::DLLExportStorageClass})
+          .to(GA);
     }
     return;
   }

test/IRGen/trivial-property-descriptors.swift

@@ -0,0 +1,10 @@
+// RUN: %swift -target x86_64-unknown-windows-msvc -parse-stdlib -module-name Swift -enable-resilience -S -emit-ir -o - %s | %FileCheck %s
+
+public struct S {}
+extension S {
+  public var i: () { return () }
+  public var j: () { return () }
+}
+
+// CHECK: @"$ss1SV1jytvpMV" = dllexport alias { i32 }, { i32 }* @"$ss1SV1iytvpMV"
+