Make it possible to build a no-exceptions variant of libcxx.

Fixes a small omission in libcxx that prevents libcxx being built when
-DLIBCXX_ENABLE_EXCEPTIONS=0 is specified.

This patch adds XFAILS to all those tests that are currently failing
on the new -fno-exceptions library variant. Follow-up patches will
update the tests (progressively) to cope with the new library variant.

Change-Id: I4b801bd8d8e4fe7193df9e55f39f1f393a8ba81a
llvm-svn: 252598
diff --git a/libcxx/test/std/experimental/string.view/string.view.access/at.pass.cpp b/libcxx/test/std/experimental/string.view/string.view.access/at.pass.cpp
index 3003ea8..a3208c1 100644
--- a/libcxx/test/std/experimental/string.view/string.view.access/at.pass.cpp
+++ b/libcxx/test/std/experimental/string.view/string.view.access/at.pass.cpp
@@ -10,6 +10,7 @@
 // NOTE: Older versions of clang have a bug where they fail to evalute
 // string_view::at as a constant expression.
 // XFAIL: clang-3.4, clang-3.3
+// XFAIL: libcpp-no-exceptions
 
 // <string_view>