Guard libc++ specific c.__invariants() tests in LIBCPP_ASSERT macros

llvm-svn: 267947
diff --git a/libcxx/test/std/strings/basic.string/string.cons/substr.pass.cpp b/libcxx/test/std/strings/basic.string/string.cons/substr.pass.cpp
index 3eaf009..a10239b 100644
--- a/libcxx/test/std/strings/basic.string/string.cons/substr.pass.cpp
+++ b/libcxx/test/std/strings/basic.string/string.cons/substr.pass.cpp
@@ -38,7 +38,7 @@
     try
     {
         S s2(str, pos);
-        assert(s2.__invariants());
+        LIBCPP_ASSERT(s2.__invariants());
         assert(pos <= str.size());
         unsigned rlen = str.size() - pos;
         assert(s2.size() == rlen);
@@ -61,7 +61,7 @@
     try
     {
         S s2(str, pos, n);
-        assert(s2.__invariants());
+        LIBCPP_ASSERT(s2.__invariants());
         assert(pos <= str.size());
         unsigned rlen = std::min<unsigned>(str.size() - pos, n);
         assert(s2.size() == rlen);
@@ -84,7 +84,7 @@
     try
     {
         S s2(str, pos, n, a);
-        assert(s2.__invariants());
+        LIBCPP_ASSERT(s2.__invariants());
         assert(pos <= str.size());
         unsigned rlen = std::min<unsigned>(str.size() - pos, n);
         assert(s2.size() == rlen);