[libcxx] Always enable availability in the lit test suite.

Summary:
Running the tests without availability enabled doesn't really make sense:
availability annotations allow catching errors at compile-time instead
of link-time. Running the tests without availability enabled allows
confirming that a test breaks at link-time under some configuration,
but it is more useful to instead check that it should fail at compile-time.

Always enabling availability in the lit test suite will greatly simplify
XFAILs and troubleshooting of failing tests, which is currently a giant
pain because we have these two levels of possible failure: link-time and
compile-time.

Reviewers: EricWF, mclow.lists

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D55079

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348296 91177308-0d34-0410-b5e6-96231b3b80d8

docs/DesignDocs/AvailabilityMarkup.rst

@@ -65,14 +65,12 @@ availability.
   the test-suite against the host system library. Alternatively a path to the
   directory containing a specific prebuilt libc++ can be used, for example:
   `--param=use_system_cxx_lib=/path/to/macOS/10.8/`.
-* The `with_availability` boolean parameter enables the availability markup.
 
 Tests can be marked as XFAIL based on multiple features made available by lit:
 
 
-* if either `use_system_cxx_lib` or `with_availability` is passed to lit,
-  assuming `--param=platform=macosx10.8` is passed as well the following
-  features will be available:
+* if `use_system_cxx_lib` is passed to lit, assuming `--param=platform=macosx10.8`
+  is passed as well the following features will be available:
 
   - availability
   - availability=x86_64