Stop using __attribute__((retain)) in GCC builds

Author: MatzeB

llvm/include/llvm/Support/Compiler.h

@@ -231,8 +231,15 @@
 #endif
 
 #if __has_attribute(retain)
+#if defined(__clang__) || !defined(__GNUC__)
 #define LLVM_ATTRIBUTE_RETAIN __attribute__((__retain__))
 #else
+// See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587
+// GCC may produce "warning: ‘retain’ attribute ignored" (despite
+// __has_attribute(retain) being 1).
+#define LLVM_ATTRIBUTE_RETAIN
+#endif
+#else
 #define LLVM_ATTRIBUTE_RETAIN
 #endif