[libc++] Use the using_if_exists attribute when provided

As discussed on cfe-dev [1], use the using_if_exists Clang attribute when
the compiler supports it. This makes it easier to port libc++ on top of
new platforms that don't fully support the C Standard library.

Previously, libc++ would fail to build when trying to import a missing
declaration in a <cXXXX> header. With the attribute, the declaration will
simply not be imported into namespace std, and hence it won't be available
for libc++ to use. In many cases, the declarations were *not* actually
required for libc++ to work (they were only surfaced for users to use
them as std::XXXX), so not importing them into namespace std is acceptable.

The same thing could be achieved by conscious usage of `#ifdef` along
with platform detection, however that quickly creates a maintenance
problem as libc++ is ported to new platforms. Furthermore, this problem
is exacerbated when mixed with vendor internal-only platforms, which can
lead to difficulties maintaining a downstream fork of the library.

For the time being, we only use the using_if_exists attribute when it
is supported. At some point in the future, we will start removing #ifdef
paths that are unnecessary when the attribute is supported, and folks
who need those #ifdef paths will be required to use a compiler that
supports the attribute.

[1]: http://lists.llvm.org/pipermail/cfe-dev/2020-June/066038.html

Differential Revision: https://reviews.llvm.org/D90257

libcxx/include/__config

@@ -880,6 +880,12 @@ typedef unsigned int   char32_t;
 #  define _LIBCPP_EXPLICIT
 #endif
 
+#if __has_attribute(using_if_exists)
+# define _LIBCPP_USING_IF_EXISTS __attribute__((using_if_exists))
+#else
+# define _LIBCPP_USING_IF_EXISTS
+#endif
+
 #ifdef _LIBCPP_HAS_NO_STRONG_ENUMS
 #  define _LIBCPP_DECLARE_STRONG_ENUM(x) struct _LIBCPP_TYPE_VIS x { enum __lx
 #  define _LIBCPP_DECLARE_STRONG_ENUM_EPILOG(x) \

libcxx/include/cctype

@@ -100,20 +100,20 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #endif
 
 
-using ::isalnum;
-using ::isalpha;
-using ::isblank;
-using ::iscntrl;
-using ::isdigit;
-using ::isgraph;
-using ::islower;
-using ::isprint;
-using ::ispunct;
-using ::isspace;
-using ::isupper;
-using ::isxdigit;
-using ::tolower;
-using ::toupper;
+using ::isalnum _LIBCPP_USING_IF_EXISTS;
+using ::isalpha _LIBCPP_USING_IF_EXISTS;
+using ::isblank _LIBCPP_USING_IF_EXISTS;
+using ::iscntrl _LIBCPP_USING_IF_EXISTS;
+using ::isdigit _LIBCPP_USING_IF_EXISTS;
+using ::isgraph _LIBCPP_USING_IF_EXISTS;
+using ::islower _LIBCPP_USING_IF_EXISTS;
+using ::isprint _LIBCPP_USING_IF_EXISTS;
+using ::ispunct _LIBCPP_USING_IF_EXISTS;
+using ::isspace _LIBCPP_USING_IF_EXISTS;
+using ::isupper _LIBCPP_USING_IF_EXISTS;
+using ::isxdigit _LIBCPP_USING_IF_EXISTS;
+using ::tolower _LIBCPP_USING_IF_EXISTS;
+using ::toupper _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 

libcxx/include/cfenv

@@ -61,20 +61,20 @@ int feupdateenv(const fenv_t* envp);
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::fenv_t;
-using ::fexcept_t;
-
-using ::feclearexcept;
-using ::fegetexceptflag;
-using ::feraiseexcept;
-using ::fesetexceptflag;
-using ::fetestexcept;
-using ::fegetround;
-using ::fesetround;
-using ::fegetenv;
-using ::feholdexcept;
-using ::fesetenv;
-using ::feupdateenv;
+using ::fenv_t _LIBCPP_USING_IF_EXISTS;
+using ::fexcept_t _LIBCPP_USING_IF_EXISTS;
+
+using ::feclearexcept _LIBCPP_USING_IF_EXISTS;
+using ::fegetexceptflag _LIBCPP_USING_IF_EXISTS;
+using ::feraiseexcept _LIBCPP_USING_IF_EXISTS;
+using ::fesetexceptflag _LIBCPP_USING_IF_EXISTS;
+using ::fetestexcept _LIBCPP_USING_IF_EXISTS;
+using ::fegetround _LIBCPP_USING_IF_EXISTS;
+using ::fesetround _LIBCPP_USING_IF_EXISTS;
+using ::fegetenv _LIBCPP_USING_IF_EXISTS;
+using ::feholdexcept _LIBCPP_USING_IF_EXISTS;
+using ::fesetenv _LIBCPP_USING_IF_EXISTS;
+using ::feupdateenv _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 

libcxx/include/cinttypes

@@ -244,13 +244,13 @@ uintmax_t wcstoumax(const wchar_t* restrict nptr, wchar_t** restrict endptr, int
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using::imaxdiv_t;
-using::imaxabs;
-using::imaxdiv;
-using::strtoimax;
-using::strtoumax;
-using::wcstoimax;
-using::wcstoumax;
+using ::imaxdiv_t _LIBCPP_USING_IF_EXISTS;
+using ::imaxabs _LIBCPP_USING_IF_EXISTS;
+using ::imaxdiv _LIBCPP_USING_IF_EXISTS;
+using ::strtoimax _LIBCPP_USING_IF_EXISTS;
+using ::strtoumax _LIBCPP_USING_IF_EXISTS;
+using ::wcstoimax _LIBCPP_USING_IF_EXISTS;
+using ::wcstoumax _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD
 

libcxx/include/clocale

@@ -43,11 +43,11 @@ lconv* localeconv();
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-using ::lconv;
+using ::lconv _LIBCPP_USING_IF_EXISTS;
 #ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
-using ::setlocale;
+using ::setlocale _LIBCPP_USING_IF_EXISTS;
 #endif
-using ::localeconv;
+using ::localeconv _LIBCPP_USING_IF_EXISTS;
 
 _LIBCPP_END_NAMESPACE_STD