Also gate AllocatedPointer and AllocAlign definitions by LLVM_VERSION_GE

Fixes a warning:

warning: llvm-wrapper/RustWrapper.cpp:159:11: warning: enumeration values 'AllocatedPointer' and 'AllocAlign' not handled in switch [-Wswitch]
warning:   switch (Kind) {
warning:           ^

Which was fall out from 130a1df.

Author: est31

compiler/rustc_llvm/llvm-wrapper/LLVMWrapper.h

@@ -87,8 +87,10 @@ enum LLVMRustAttribute {
   NoCfCheck = 35,
   ShadowCallStack = 36,
   AllocSize = 37,
+#if LLVM_VERSION_GE(15, 0)
   AllocatedPointer = 38,
   AllocAlign = 39,
+#endif
 };
 
 typedef struct OpaqueRustString *RustStringRef;