[libc] Move __LLVM_LIBC__ define to __llvm-libc-common.h (llvm#126877)

Relying on features.h is problematic since codebases are free to have
such a header on their search path, which breaks compilation. libc
should instead provide a more standard way of getting __LLVM_LIBC__.
Since __llvm-libc-common.h is included from all libc headers, defining
__LLVM_LIBC__ there ensures that this define is available whenever any
of the standard header is included.

(cherry picked from commit b0d7820)

Author: petrhosek

libc/include/__llvm-libc-common.h

@@ -9,6 +9,8 @@
 #ifndef LLVM_LIBC_COMMON_H
 #define LLVM_LIBC_COMMON_H
 
+#define __LLVM_LIBC__ 1
+
 #ifdef __cplusplus
 
 #undef __BEGIN_C_DECLS

libc/include/llvm-libc-macros/features-macros.h

@@ -9,6 +9,4 @@
 #ifndef LLVM_LIBC_MACROS_FEATURES_MACROS_H
 #define LLVM_LIBC_MACROS_FEATURES_MACROS_H
 
-#define __LLVM_LIBC__ 1
-
 #endif // LLVM_LIBC_MACROS_FEATURES_MACROS_H