[AllocToken] Introduce sanitize_alloc_token attribute and alloc_token metadata (#160131)

In preparation of adding the "AllocToken" pass, add the pre-requisite
`sanitize_alloc_token` function attribute and `alloc_token` metadata.

---

This change is part of the following series:
  1. llvm/llvm-project#160131
  2. llvm/llvm-project#156838
  3. llvm/llvm-project#162098
  4. llvm/llvm-project#162099
  5. llvm/llvm-project#156839
  6. llvm/llvm-project#156840
  7. llvm/llvm-project#156841
  8. llvm/llvm-project#156842

Author: melver

syntax/llvm.vim

@@ -163,6 +163,7 @@ syn keyword llvmKeyword
       \ returns_twice
       \ safestack
       \ sanitize_address
+      \ sanitize_alloc_token
       \ sanitize_hwaddress
       \ sanitize_memory
       \ sanitize_memtag