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/strings/basic.string/string.capacity/reserve.pass.cpp b/libcxx/test/std/strings/basic.string/string.capacity/reserve.pass.cpp
index a155825..569ca76 100644
--- a/libcxx/test/std/strings/basic.string/string.capacity/reserve.pass.cpp
+++ b/libcxx/test/std/strings/basic.string/string.capacity/reserve.pass.cpp
@@ -7,6 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: libcpp-no-exceptions
 // <string>
 
 // void reserve(size_type res_arg=0);