fix TryToFixInvalidVariablyModifiedType to reject negative array sizes

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63557 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/Sema/struct-decl.c b/test/Sema/struct-decl.c
index cacd847..7d7961b 100644
--- a/test/Sema/struct-decl.c
+++ b/test/Sema/struct-decl.c
@@ -7,4 +7,5 @@
 
 struct foo {
 	char name[(int)&((struct bar *)0)->n];
+	char name2[(int)&((struct bar *)0)->n - 1]; //expected-error{{fields must have a constant size}}
 };