[IRGen] Correctly assign lazily-emitted global variables in multi-threaded IRGen

With multi-threaded IRGen, the global variables associated with "once"
initialization tokens were not getting colocated with their actual
global variables, which caused the initialization code to get split
across different files. This issue manifest as autolinking errors in
some projects.

Fixes rdar://162400654.

test/IRGen/multithread_global_var.swift

@@ -0,0 +1,24 @@
+// RUN: %empty-directory(%t/src)
+// RUN: split-file %s %t/src
+
+// RUN: %target-swift-frontend %t/src/A.swift %t/src/B.swift -emit-ir -o %t/A.ll -o %t/B.ll -num-threads 2 -O -g -module-name test
+// RUN: %FileCheck --check-prefix=CHECK-A %s <%t/A.ll
+// RUN: %FileCheck --check-prefix=CHECK-B %s <%t/B.ll
+
+//--- A.swift
+
+public func f() -> String { "hello" }
+
+public func g() -> Bool {
+  f() == X.introduction
+}
+
+// CHECK-A: @"$s4test1XV12introduction_Wz" = external hidden global
+
+//--- B.swift
+
+public struct X {
+  public static var introduction: String = f().uppercased()
+}
+
+// CHECK-B: @"$s4test1XV12introduction_Wz" = weak_odr hidden global