[libc] Add LIBC_NAMESPACE_DECL macro

This defines to LIBC_NAMESPACE with
`__attribute__((visibility("hidden")))` so all the symbols under it have
hidden visibility. This new macro should be used when declaring a new
namespace that will have internal functions/globals and LIBC_NAMESPACE
should be used as a means of accessing functions/globals declared within
LIBC_NAMESPACE_DECL.

Author: PiJoules

libc/docs/dev/code_style.rst

@@ -260,3 +260,34 @@ Patches containing any amount of Assembly ideally should be approved by 2
 maintainers. llvm-libc maintainers reserve the right to reject Assembly
 contributions that they feel could be better maintained if rewritten in C++,
 and to revisit this policy in the future.
+
+LIBC_NAMESPACE_DECL
+===================
+
+llvm-libc provides a macro `LIBC_NAMESPACE` which contains internal implementations of
+libc functions and globals. This macro should only be used as an
+identifier for accessing such symbols within the namespace (like `LIBC_NAMESPACE::cpp::max`).
+Any usage of this namespace for declaring or defining internal symbols should
+instead use `LIBC_NAMESPACE_DECL` which declares `LIBC_NAMESPACE` with hidden visibility.
+
+Example usage:
+
+.. code-block:: c++
+
+   #include "src/__support/macros/config.h"  // The macro is defined here.
+
+   namespace LIBC_NAMESPACE_DECL {
+
+   void new_function() {
+     ...
+   }
+
+   }  // LIBC_NAMESPACE_DECL
+
+Having hidden visibility on the namespace ensures etern declarations in a given TU
+have known visibility and never generate GOT indirextions. The attribute guarantees
+this independently of global compile options and build systems.
+
+..
+  TODO(leonardchan): We should have a clang-tidy check to enforce this and a
+  fixit implementation.

libc/src/__support/macros/config.h

@@ -27,4 +27,12 @@
 #define LIBC_HAS_FEATURE(f) 0
 #endif
 
+// Declare a LIBC_NAMESPACE with hidden visibility. `namespace
+// LIBC_NAMESPACE_DECL {` should be used around all declarations and definitions
+// for libc internals as opposed to just `namespace LIBC_NAMESPACE {`. This
+// ensures that all internal implementations within this namespace have hidden
+// visibility. This does not affect the public C symbols which are controlled
+// independently via `LLVM_LIBC_FUNCTION_ATTR`.
+#define LIBC_NAMESPACE_DECL [[gnu::visibility("hidden")]] LIBC_NAMESPACE
+
 #endif // LLVM_LIBC_SRC___SUPPORT_MACROS_CONFIG_H