[libc] Add LIBC_INLINE_VAR for inline variables

[abrachet]

Summary

These are the only variables I could find that use LIBC_INLINE. Note, these are namespace scoped constexpr so local linkage is implied. inline is useful here to silence clang's unused-const-variable variable. For Fuchsia, the distinction between LIBC_INLINE and LIBC_INLINE_VAR is helpful because we define LIBC_INLINE as [[gnu::always_inline]] inline when building with gcc. This isn't meaningful on variables.

Alternatively, we could make these variables simply constexpr and also add [[maybe_unused]]

[mcgrathr]

LGTM but maybe there's some doc pages somewhere that should be updated to mention using LIBC_INLINE_VAR.

[sivachandra]

Sorry, I missed this because it was not copied to libc-commits.
+1 for documentation - a section similar to this may be: https://libc.llvm.org/dev/code_style.html#inline-functions-defined-in-header-files

libc/docs/dev/code_style.rst

#. **Code defined** macros are defined within the ``src/__support/macros``
    * **Properties** - Build related properties like used compiler, target
      architecture or enabled CPU features defined by introspecting compiler
      defined preprocessor definitions. e.g., ``LIBC_TARGET_ARCH_IS_ARM``,
      ``LIBC_TARGET_CPU_HAS_AVX2``, ``LIBC_COMPILER_IS_CLANG``, ...
    * **Attributes** - Compiler agnostic attributes or functions to handle
      specific operations. e.g., ``LIBC_INLINE``, ``LIBC_NO_LOOP_UNROLL``,
      ``LIBC_LIKELY``, ``LIBC_INLINE_ASM``.
 
-Inline functions defined in header files
+Inline functions and variables defined in header files
 ========================================
 
-When defining functions inline in header files, we follow certain rules:
+When defining functions and variables inline in header files, we follow certain
+rules:
 
 #. The functions should not be given file-static linkage. There can be class
    static methods defined inline however.
-#. Instead of using the ``inline`` keyword, they should be tagged with the
-   ``LIBC_INLINE`` macro defined in ``src/__support/common.h``. For example:
+#. Instead of using the ``inline`` keyword, functions should be tagged with the
+   ``LIBC_INLINE`` macro and variables should be tagged with the
+   ``LIBC_INLINE_VAR`` macro defined in ``src/__support/macros/attributes.h``.
+   For example:
 
    .. code-block:: c++
 
+     LIBC_INLINE_VAR constexpr bool foo = true;
+
      LIBC_INLINE ReturnType function_defined_inline(ArgType arg) {
        ...
      }
 
 #. The ``LIBC_INLINE`` tag should also be added to functions which have
    definitions that are implicitly inline. Examples of such functions are
    class methods (static and non-static) defined inline and ``constexpr``
    functions.

libc/src/__support/macros/attributes.h

 // other compilers.
 
 #ifndef LLVM_LIBC_SUPPORT_MACROS_ATTRIBUTES_H
 #define LLVM_LIBC_SUPPORT_MACROS_ATTRIBUTES_H
 
 #include "properties/architectures.h"
 
 #define LIBC_INLINE inline
+#define LIBC_INLINE_VAR inline
 #define LIBC_INLINE_ASM __asm__ __volatile__
 #define LIBC_UNUSED __attribute__((unused))
 
 #ifdef LIBC_TARGET_ARCH_IS_GPU
 #define LIBC_THREAD_LOCAL
 #else
 #define LIBC_THREAD_LOCAL thread_local
 #endif
 
 #endif // LLVM_LIBC_SUPPORT_MACROS_ATTRIBUTES_H

libc/src/string/memory_utils/op_x86.h

 #endif
 #if !defined(__SSE2__) && (defined(_MSC_VER) || defined(__SCE__))
 #define _mm_movemask_epi8(A) 0
 #endif
 
 namespace __llvm_libc::x86 {
 
 // A set of constants to check compile time features.
-LIBC_INLINE static constexpr bool kSse2 = LLVM_LIBC_IS_DEFINED(__SSE2__);
-LIBC_INLINE static constexpr bool kSse41 = LLVM_LIBC_IS_DEFINED(__SSE4_1__);
-LIBC_INLINE static constexpr bool kAvx = LLVM_LIBC_IS_DEFINED(__AVX__);
-LIBC_INLINE static constexpr bool kAvx2 = LLVM_LIBC_IS_DEFINED(__AVX2__);
-LIBC_INLINE static constexpr bool kAvx512F = LLVM_LIBC_IS_DEFINED(__AVX512F__);
-LIBC_INLINE static constexpr bool kAvx512BW =
-    LLVM_LIBC_IS_DEFINED(__AVX512BW__);
+LIBC_INLINE_VAR constexpr bool kSse2 = LLVM_LIBC_IS_DEFINED(__SSE2__);
+LIBC_INLINE_VAR constexpr bool kSse41 = LLVM_LIBC_IS_DEFINED(__SSE4_1__);
+LIBC_INLINE_VAR constexpr bool kAvx = LLVM_LIBC_IS_DEFINED(__AVX__);
+LIBC_INLINE_VAR constexpr bool kAvx2 = LLVM_LIBC_IS_DEFINED(__AVX2__);
+LIBC_INLINE_VAR constexpr bool kAvx512F = LLVM_LIBC_IS_DEFINED(__AVX512F__);
+LIBC_INLINE_VAR constexpr bool kAvx512BW = LLVM_LIBC_IS_DEFINED(__AVX512BW__);
 
 ///////////////////////////////////////////////////////////////////////////////
 // Memcpy repmovsb implementation
 struct Memcpy {
   static void repmovsb(void *dst, const void *src, size_t count) {
     asm volatile("rep movsb" : "+D"(dst), "+S"(src), "+c"(count) : : "memory");
   }
 };