MC: Emit COFF section flags in the "proper" order

COFF section flags are not idempotent:
  'rd' will make a read-write section because 'd' implies write
  'dr' will make a read-only section because 'r' disables write

llvm-svn: 228490
diff --git a/llvm/test/MC/COFF/initialised-data.ll b/llvm/test/MC/COFF/initialised-data.ll
index c428469..a2faac7 100644
--- a/llvm/test/MC/COFF/initialised-data.ll
+++ b/llvm/test/MC/COFF/initialised-data.ll
@@ -3,5 +3,5 @@
 
 @data = dllexport constant [5 x i8] c"data\00", align 1
 
-; CHECK: .section	.rdata,"rd"
+; CHECK: .section	.rdata,"dr"