Add markup for libc++ dylib availability

Libc++ is used as a system library on macOS and iOS (amongst others). In order
for users to be able to compile a binary that is intended to be deployed to an
older version of the platform, clang provides the
availability attribute <https://clang.llvm.org/docs/AttributeReference.html#availability>_
that can be placed on declarations to describe the lifecycle of a symbol in the
library.

See docs/DesignDocs/AvailabilityMarkup.rst for more information.

Differential Revision: https://reviews.llvm.org/D31739

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302172 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/language.support/support.exception/uncaught/uncaught_exceptions.pass.cpp

@@ -8,6 +8,14 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: libcpp-no-exceptions
+// XFAIL: libcpp-no-exceptions
+
+// XFAIL: availability=macosx10.7
+// XFAIL: availability=macosx10.8
+// XFAIL: availability=macosx10.9
+// XFAIL: availability=macosx10.10
+// XFAIL: availability=macosx10.11
+
 // test uncaught_exceptions
 
 #include <exception>