[test][IRGen] Maybe Windows needs dso_local.

In swiftlang#33816, we forked the local_extern test omitting the check for the
extern function, because it wasn't obvious what the match was. It might
actually just be missing the dso_local annotation again for the
function. Let's try that.

Author: 3405691582

test/IRGen/local_extern.swift

@@ -1,8 +1,7 @@
-// XFAIL: OS=windows-msvc
 // RUN: %target-swift-frontend -import-objc-header %S/Inputs/local_extern.h %s -emit-ir | %FileCheck %s
 // CHECK: @var = external {{(dso_local )?}}global i32
 // CHECK: @prior_var = internal global i32
-// CHECK: declare i32 @func
+// CHECK: declare {{(dso_local )?}}i32 @func
 // CHECK: define internal i32 @prior_func
 
 print("\(_no_prior_var())")