Adjust libc++ test infastructure to fully support modules

This patch overhalls the libc++ test format/configuration in order to fully support modules. By "fully support" I mean get almost all of the tests passing. The main hurdle for doing this is handling tests that `#define _LIBCPP_FOO` macros to test a different configuration. This patch deals with these tests in the following ways:

1. For tests that define single `_LIBCPP_ABI_FOO` macros have been annotated with `// MODULES_DEFINES: _LIBCPP_ABI_FOO`. This allows the test suite to define the macro on the command line so it uses a different set of modules.
2. Tests for libc++'s debug mode (which define custom `_LIBCPP_ASSERT`) are automatically detected by the test suite and are compiled and run with modules disabled.

This patch also cleans up how the `CXXCompiler` helper class handles enabling/disabling language features.

NOTE: This patch uses `LIT` features which were only committed to LLVM today. If this patch breaks running the libc++ tests you probably need to update LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@288728 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/utilities/utility/pairs/pairs.pair/non_trivial_copy_move_ABI.pass.cpp

@@ -10,6 +10,10 @@
 // The test fails due to the missing is_trivially_constructible intrinsic.
 // XFAIL: gcc-4.9
 
+// The test suite needs to define the ABI macros on the command line when
+// modules are enabled.
+// UNSUPPORTED: -fmodules
+
 // <utility>
 
 // template <class T1, class T2> struct pair