[libc++] Add _LIBCPP_DISABLE_EXTERN_TEMPLATE config option

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.

Differential Revision: https://reviews.llvm.org/D31725

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300246 91177308-0d34-0410-b5e6-96231b3b80d8

docs/UsingLibcxx.rst

@@ -146,6 +146,11 @@ thread safety annotations.
   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

include/__config

@@ -856,6 +856,11 @@ template <unsigned> struct __static_assert_check {};
 # 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