Add __attribute__((retain)) to LLVM_DUMP_METHOD (#133025)

Without the retain attribute the dump functions will be stripped when
LLVM is compiled with `-ffunction-section -Wl,--gc-sections` on
ELF-based systems.

Author: MatzeB

llvm/include/llvm/Support/Compiler.h

@@ -230,6 +230,12 @@
 #define LLVM_ATTRIBUTE_USED
 #endif
 
+#if __has_attribute(retain)
+#define LLVM_ATTRIBUTE_RETAIN __attribute__((__retain__))
+#else
+#define LLVM_ATTRIBUTE_RETAIN
+#endif
+
 #if defined(__clang__)
 #define LLVM_DEPRECATED(MSG, FIX) __attribute__((deprecated(MSG, FIX)))
 #else
@@ -619,7 +625,8 @@ void AnnotateIgnoreWritesEnd(const char *file, int line);
 /// get stripped in release builds.
 // FIXME: Move this to a private config.h as it's not usable in public headers.
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
-#define LLVM_DUMP_METHOD LLVM_ATTRIBUTE_NOINLINE LLVM_ATTRIBUTE_USED
+#define LLVM_DUMP_METHOD                                                       \
+  LLVM_ATTRIBUTE_NOINLINE LLVM_ATTRIBUTE_USED LLVM_ATTRIBUTE_RETAIN
 #else
 #define LLVM_DUMP_METHOD LLVM_ATTRIBUTE_NOINLINE
 #endif