[libunwind] Remove static_assert / __has_feature macros

The static_assert macro broke on GCC when a scope had two asserts and a
condition that depended on a template parameter. Remove the macro and
rely on the compiler's C++11 static_assert feature.

The __has_feature macro was only used here to determine whether to
define the static_assert macro.

Reviewed By: mstorsjo, #libunwind

Differential Revision: https://reviews.llvm.org/D86411

libunwind/src/config.h

@@ -18,16 +18,6 @@
 #include <stdint.h>
 #include <stdlib.h>
 
-// Define static_assert() unless already defined by compiler.
-#ifndef __has_feature
-  #define __has_feature(__x) 0
-#endif
-#if !(__has_feature(cxx_static_assert)) && !defined(static_assert)
-  #define static_assert(__b, __m) \
-      extern int compile_time_assert_failed[ ( __b ) ? 1 : -1 ]  \
-                                                  __attribute__( ( unused ) );
-#endif
-
 // Platform specific configuration defines.
 #ifdef __APPLE__
   #if defined(FOR_DYLD)