C/C++ front-end: declare __builtin_is_constant_evaluated

GCC built-in to implement std::is_constant_evaluated. We just provide a
declaration, but no implementation at this point.

Author: tautschnig

src/ansi-c/compiler_headers/gcc_builtin_headers_generic.h

@@ -44,6 +44,7 @@ long __builtin_expect_with_probability(long, long, double);
 void __builtin_clear_padding();
 void __builtin_speculation_safe_value();
 void* __builtin_speculation_safe_value_ptr(void*, ...);
+_Bool __builtin_is_constant_evaluated(void);
 
 void* __builtin_dwarf_cfa();
 unsigned __builtin_dwarf_sp_column();