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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@252598 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/thread/thread.mutex/thread.lock.algorithm/try_lock.pass.cpp

@@ -7,6 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
+// XFAIL: libcpp-no-exceptions
 // UNSUPPORTED: libcpp-has-no-threads
 
 // <mutex>