Add _LIBCPP_BUILTIN_CONSTANT_P support.

[mvels]

Summary

This change adds the macros _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P and _LIBCPP_BUILTIN_CONSTANT_P to detect compile time constants, and optimze the code accordingly.

A planned usage example:
The implementation of basic_string::assign() can short-cut a compile time known short string assignent into a fast and compact inlined assignment:

basic_string::assign(const value_type* __s) {
  if (_LIBCPP_BUILTIN_CONSTANT_P(__s[0]) && length(__s) < __min_cap) {
    copy(pointer(), _s, length(__s) + 1);
    set_size(length(__s));
  } else {
    // delegate / tail call out of line implementation
  }
}

[EricWF]

libcxx/include/__config

546

Just put this in the same block as the other feature check for the same version.

[mvels]

Combined ifdef block for GNUC_VER

libcxx/include/__config

 #if !defined(_LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK) && __has_attribute(no_sanitize)
 #define _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK __attribute__((__no_sanitize__("unsigned-integer-overflow")))
 #endif
 
 #if __has_builtin(__builtin_launder)
 #define _LIBCPP_COMPILER_HAS_BUILTIN_LAUNDER
 #endif
 
+#if __has_builtin(__builtin_constant_p)
+#define _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
+#endif
+
 #if !__is_identifier(__has_unique_object_representations)
 #define _LIBCPP_HAS_UNIQUE_OBJECT_REPRESENTATIONS
 #endif
 
 #define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
 
 // Literal operators ""d and ""y are supported starting with LLVM Clang 8 and AppleClang 10.0.1
 #if (defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER < 800) || \

 #endif
 
 #if !defined(_LIBCPP_HAS_NO_ASAN) && !defined(__SANITIZE_ADDRESS__)
 #define _LIBCPP_HAS_NO_ASAN
 #endif
 
 #if _GNUC_VER >= 700
 #define _LIBCPP_COMPILER_HAS_BUILTIN_LAUNDER
-#endif
-
-#if _GNUC_VER >= 700
+#define _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
 #define _LIBCPP_HAS_UNIQUE_OBJECT_REPRESENTATIONS
 #endif
 
 #define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
 
 #define _LIBCPP_DISABLE_EXTENSION_WARNING __extension__
 
 #elif defined(_LIBCPP_COMPILER_MSVC)
 
 #define _LIBCPP_TOSTRING2(x) #x

 // Not all platforms support this, but it helps avoid fd-leaks on platforms that
 // do.
 #if defined(__BIONIC__)
 #  define _LIBCPP_FOPEN_CLOEXEC_MODE "e"
 #else
 #  define _LIBCPP_FOPEN_CLOEXEC_MODE
 #endif
 
+#ifdef _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
+#define _LIBCPP_BUILTIN_CONSTANT_P(x) __builtin_constant_p(x)
+#else
+#define _LIBCPP_BUILTIN_CONSTANT_P(x) false
+#endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG