Stop using __attribute__((retain)) in GCC builds (#133793)

GCC sometimes produces warnings about `__attribute__((retain))` despite
`__has_attribute(retain)` being 1. See:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587

The amount of users who benefit from the attribute is probably very
small compared to the amount of `-Werror` enabled builds or the desire
to keep `-Wattributes` enabled in the LLVM build. So for now drop usage
of the `retain` attribute in GCC builds.

Author: MatzeB

llvm/include/llvm/Support/Compiler.h

@@ -230,7 +230,11 @@
 #define LLVM_ATTRIBUTE_USED
 #endif
 
-#if __has_attribute(retain)
+// Only enabled for clang:
+// See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587
+// GCC may produce "warning: ‘retain’ attribute ignored" (despite
+// __has_attribute(retain) being 1).
+#if defined(__clang__) && __has_attribute(retain)
 #define LLVM_ATTRIBUTE_RETAIN __attribute__((__retain__))
 #else
 #define LLVM_ATTRIBUTE_RETAIN