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/thread/thread.mutex/thread.lock/thread.lock.guard/variadic_mutex_mangling.pass.cpp

@@ -23,6 +23,7 @@
 // C++11 and C++03. This is important since the mangling of `lock_guard` depends
 // on it being declared as a variadic template, even in C++03.
 
+// MODULES_DEFINES: _LIBCPP_ABI_VARIADIC_LOCK_GUARD
 #define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
 #include <mutex>
 #include <string>

test/libcxx/thread/thread.mutex/thread_safety_lock_guard.pass.cpp

@@ -12,6 +12,7 @@
 
 // <mutex>
 
+// MODULES_DEFINES: _LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS
 #define _LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS
 
 #include <mutex>

test/libcxx/thread/thread.mutex/thread_safety_lock_unlock.pass.cpp

@@ -12,6 +12,7 @@
 
 // <mutex>
 
+// MODULES_DEFINES: _LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS
 #define _LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS
 
 #include <mutex>

test/libcxx/thread/thread.mutex/thread_safety_missing_unlock.fail.cpp

@@ -12,6 +12,7 @@
 
 // <mutex>
 
+// MODULES_DEFINES: _LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS
 #define _LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS
 
 #include <mutex>

test/libcxx/thread/thread.mutex/thread_safety_requires_capability.pass.cpp

@@ -12,6 +12,7 @@
 
 // <mutex>
 
+// MODULES_DEFINES: _LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS
 #define _LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS
 
 #include <mutex>