[libc++] Add _LIBCPP_DISABLE_EXTERN_TEMPLATE config option

[smeenai]

Summary

When the libc++ extern template macros were added, the intent was for it
to be possible for consumers of the headers to disable extern templates
(via -D_LIBCPP_EXTERN_TEMPLATE(...)=). Unfortunately, support for
specifying function-like macros varies on the command line varies across
compilers (e.g. MSVC doesn't support it at all), and cmake doesn't allow
it for the same reason. Add a non-function macro for this purpose.

The intended use is for libraries which want to use the libc++ headers
without taking a dependency on the libc++ library itself. I can name the
macro something which reflects its intent rather than its behavior (e.g.
_LIBCPP_HEADER_ONLY) if desired.

[EricWF]

Please add docs to http://libcxx.llvm.org/docs/UsingLibcxx.html#libc-configuration-macros

[EricWF]

LGTM minus doc.

[smeenai]

Add documentation

libcxx/trunk/docs/UsingLibcxx.rst

**_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS**:
   disabled and must be manually enabled by the user.
 
 **_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS**:
   This macro is used to disable all visibility annotations inside libc++.
   Defining this macro and then building libc++ with hidden visibility gives a
   build of libc++ which does not export any symbols, which can be useful when
   building statically for inclusion into another library.
 
+**_LIBCPP_DISABLE_EXTERN_TEMPLATE**:
+  This macro is used to disable extern template declarations in the libc++
+  headers. The intended use case is for clients who wish to use the libc++
+  headers without taking a dependency on the libc++ library itself.
+
 **_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION**:
   This macro is used to re-enable an extension in `std::tuple` which allowed
   it to be implicitly constructed from fewer initializers than contained
   elements. Elements without an initializer are default constructed. For example:
 
   .. code-block:: cpp
 
     std::tuple<std::string, int, std::error_code> foo() {

libcxx/trunk/include/__config

 #   else
 #       error Supported values for _LIBCPP_DEBUG are 0 and 1
 #   endif
 # if !defined(_LIBCPP_BUILDING_LIBRARY)
 #   define _LIBCPP_EXTERN_TEMPLATE(...)
 # endif
 #endif
 
+#ifdef _LIBCPP_DISABLE_EXTERN_TEMPLATE
+#define _LIBCPP_EXTERN_TEMPLATE(...)
+#define _LIBCPP_EXTERN_TEMPLATE2(...)
+#endif
+
 #ifndef _LIBCPP_EXTERN_TEMPLATE
 #define _LIBCPP_EXTERN_TEMPLATE(...) extern template __VA_ARGS__;
 #endif
 
 #ifndef _LIBCPP_EXTERN_TEMPLATE2
 #define _LIBCPP_EXTERN_TEMPLATE2(...) extern template __VA_ARGS__;
 #endif