Ensure that we don't diagnose the lack of initializer for @_extern variables

Author: DougGregor

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2589,6 +2589,10 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
         if (!var->hasStorage())
           return;
 
+        // If the variable is @_extern, it never needs an initializer.
+        if (var->getAttrs().hasAttribute<ExternAttr>())
+          return;
+
         if (var->getAttrs().hasAttribute<SILGenNameAttr>()
               || !ABIRoleInfo(var).providesAPI())
           return;

test/attr/attr_extern.swift

@@ -73,6 +73,9 @@ struct StructScopeC {
 
     @_extern(c, "static_member_decl")
     static func staticMemberDecl()
+
+    @_extern(c, "global_static_variable")
+    static var globalVariable: Int
 }
 
 struct GenericStruct<T> {